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diff --git a/Doc/howto/functional.rst b/Doc/howto/functional.rst new file mode 100644 index 0000000000..8f8afac233 --- /dev/null +++ b/Doc/howto/functional.rst @@ -0,0 +1,1279 @@ +Functional Programming HOWTO +================================ + +**Version 0.21** + +(This is a first draft. Please send comments/error +reports/suggestions to amk@amk.ca. This URL is probably not going to +be the final location of the document, so be careful about linking to +it -- you may want to add a disclaimer.) + +In this document, we'll take a tour of Python's features suitable for +implementing programs in a functional style. After an introduction to +the concepts of functional programming, we'll look at language +features such as iterators and generators and relevant library modules +such as ``itertools`` and ``functools``. + + +Introduction +---------------------- + +This section explains the basic concept of functional programming; if +you're just interested in learning about Python language features, +skip to the next section. + +Programming languages support decomposing problems in several different +ways: + +* Most programming languages are **procedural**: + programs are lists of instructions that tell the computer what to + do with the program's input. + C, Pascal, and even Unix shells are procedural languages. + +* In **declarative** languages, you write a specification that describes + the problem to be solved, and the language implementation figures out + how to perform the computation efficiently. SQL is the declarative + language you're most likely to be familiar with; a SQL query describes + the data set you want to retrieve, and the SQL engine decides whether to + scan tables or use indexes, which subclauses should be performed first, + etc. + +* **Object-oriented** programs manipulate collections of objects. + Objects have internal state and support methods that query or modify + this internal state in some way. Smalltalk and Java are + object-oriented languages. C++ and Python are languages that + support object-oriented programming, but don't force the use + of object-oriented features. + +* **Functional** programming decomposes a problem into a set of functions. + Ideally, functions only take inputs and produce outputs, and don't have any + internal state that affects the output produced for a given input. + Well-known functional languages include the ML family (Standard ML, + OCaml, and other variants) and Haskell. + +The designers of some computer languages have chosen one approach to +programming that's emphasized. This often makes it difficult to +write programs that use a different approach. Other languages are +multi-paradigm languages that support several different approaches. Lisp, +C++, and Python are multi-paradigm; you can write programs or +libraries that are largely procedural, object-oriented, or functional +in all of these languages. In a large program, different sections +might be written using different approaches; the GUI might be object-oriented +while the processing logic is procedural or functional, for example. + +In a functional program, input flows through a set of functions. Each +function operates on its input and produces some output. Functional +style frowns upon functions with side effects that modify internal +state or make other changes that aren't visible in the function's +return value. Functions that have no side effects at all are +called **purely functional**. +Avoiding side effects means not using data structures +that get updated as a program runs; every function's output +must only depend on its input. + +Some languages are very strict about purity and don't even have +assignment statements such as ``a=3`` or ``c = a + b``, but it's +difficult to avoid all side effects. Printing to the screen or +writing to a disk file are side effects, for example. For example, in +Python a ``print`` statement or a ``time.sleep(1)`` both return no +useful value; they're only called for their side effects of sending +some text to the screen or pausing execution for a second. + +Python programs written in functional style usually won't go to the +extreme of avoiding all I/O or all assignments; instead, they'll +provide a functional-appearing interface but will use non-functional +features internally. For example, the implementation of a function +will still use assignments to local variables, but won't modify global +variables or have other side effects. + +Functional programming can be considered the opposite of +object-oriented programming. Objects are little capsules containing +some internal state along with a collection of method calls that let +you modify this state, and programs consist of making the right set of +state changes. Functional programming wants to avoid state changes as +much as possible and works with data flowing between functions. In +Python you might combine the two approaches by writing functions that +take and return instances representing objects in your application +(e-mail messages, transactions, etc.). + +Functional design may seem like an odd constraint to work under. Why +should you avoid objects and side effects? There are theoretical and +practical advantages to the functional style: + +* Formal provability. +* Modularity. +* Composability. +* Ease of debugging and testing. + +Formal provability +'''''''''''''''''''''' + +A theoretical benefit is that it's easier to construct a mathematical proof +that a functional program is correct. + +For a long time researchers have been interested in finding ways to +mathematically prove programs correct. This is different from testing +a program on numerous inputs and concluding that its output is usually +correct, or reading a program's source code and concluding that the +code looks right; the goal is instead a rigorous proof that a program +produces the right result for all possible inputs. + +The technique used to prove programs correct is to write down +**invariants**, properties of the input data and of the program's +variables that are always true. For each line of code, you then show +that if invariants X and Y are true **before** the line is executed, +the slightly different invariants X' and Y' are true **after** +the line is executed. This continues until you reach the end of the +program, at which point the invariants should match the desired +conditions on the program's output. + +Functional programming's avoidance of assignments arose because +assignments are difficult to handle with this technique; +assignments can break invariants that were true before the assignment +without producing any new invariants that can be propagated onward. + +Unfortunately, proving programs correct is largely impractical and not +relevant to Python software. Even trivial programs require proofs that +are several pages long; the proof of correctness for a moderately +complicated program would be enormous, and few or none of the programs +you use daily (the Python interpreter, your XML parser, your web +browser) could be proven correct. Even if you wrote down or generated +a proof, there would then be the question of verifying the proof; +maybe there's an error in it, and you wrongly believe you've proved +the program correct. + +Modularity +'''''''''''''''''''''' + +A more practical benefit of functional programming is that it forces +you to break apart your problem into small pieces. Programs are more +modular as a result. It's easier to specify and write a small +function that does one thing than a large function that performs a +complicated transformation. Small functions are also easier to read +and to check for errors. + + +Ease of debugging and testing +'''''''''''''''''''''''''''''''''' + +Testing and debugging a functional-style program is easier. + +Debugging is simplified because functions are generally small and +clearly specified. When a program doesn't work, each function is an +interface point where you can check that the data are correct. You +can look at the intermediate inputs and outputs to quickly isolate the +function that's responsible for a bug. + +Testing is easier because each function is a potential subject for a +unit test. Functions don't depend on system state that needs to be +replicated before running a test; instead you only have to synthesize +the right input and then check that the output matches expectations. + + + +Composability +'''''''''''''''''''''' + +As you work on a functional-style program, you'll write a number of +functions with varying inputs and outputs. Some of these functions +will be unavoidably specialized to a particular application, but +others will be useful in a wide variety of programs. For example, a +function that takes a directory path and returns all the XML files in +the directory, or a function that takes a filename and returns its +contents, can be applied to many different situations. + +Over time you'll form a personal library of utilities. Often you'll +assemble new programs by arranging existing functions in a new +configuration and writing a few functions specialized for the current +task. + + + +Iterators +----------------------- + +I'll start by looking at a Python language feature that's an important +foundation for writing functional-style programs: iterators. + +An iterator is an object representing a stream of data; this object +returns the data one element at a time. A Python iterator must +support a method called ``next()`` that takes no arguments and always +returns the next element of the stream. If there are no more elements +in the stream, ``next()`` must raise the ``StopIteration`` exception. +Iterators don't have to be finite, though; it's perfectly reasonable +to write an iterator that produces an infinite stream of data. + +The built-in ``iter()`` function takes an arbitrary object and tries +to return an iterator that will return the object's contents or +elements, raising ``TypeError`` if the object doesn't support +iteration. Several of Python's built-in data types support iteration, +the most common being lists and dictionaries. An object is called +an **iterable** object if you can get an iterator for it. + +You can experiment with the iteration interface manually:: + + >>> L = [1,2,3] + >>> it = iter(L) + >>> print it + <iterator object at 0x8116870> + >>> it.next() + 1 + >>> it.next() + 2 + >>> it.next() + 3 + >>> it.next() + Traceback (most recent call last): + File "<stdin>", line 1, in ? + StopIteration + >>> + +Python expects iterable objects in several different contexts, the +most important being the ``for`` statement. In the statement ``for X in Y``, +Y must be an iterator or some object for which ``iter()`` can create +an iterator. These two statements are equivalent:: + + for i in iter(obj): + print i + + for i in obj: + print i + +Iterators can be materialized as lists or tuples by using the +``list()`` or ``tuple()`` constructor functions:: + + >>> L = [1,2,3] + >>> iterator = iter(L) + >>> t = tuple(iterator) + >>> t + (1, 2, 3) + +Sequence unpacking also supports iterators: if you know an iterator +will return N elements, you can unpack them into an N-tuple:: + + >>> L = [1,2,3] + >>> iterator = iter(L) + >>> a,b,c = iterator + >>> a,b,c + (1, 2, 3) + +Built-in functions such as ``max()`` and ``min()`` can take a single +iterator argument and will return the largest or smallest element. +The ``"in"`` and ``"not in"`` operators also support iterators: ``X in +iterator`` is true if X is found in the stream returned by the +iterator. You'll run into obvious problems if the iterator is +infinite; ``max()``, ``min()``, and ``"not in"`` will never return, and +if the element X never appears in the stream, the ``"in"`` operator +won't return either. + +Note that you can only go forward in an iterator; there's no way to +get the previous element, reset the iterator, or make a copy of it. +Iterator objects can optionally provide these additional capabilities, +but the iterator protocol only specifies the ``next()`` method. +Functions may therefore consume all of the iterator's output, and if +you need to do something different with the same stream, you'll have +to create a new iterator. + + + +Data Types That Support Iterators +''''''''''''''''''''''''''''''''''' + +We've already seen how lists and tuples support iterators. In fact, +any Python sequence type, such as strings, will automatically support +creation of an iterator. + +Calling ``iter()`` on a dictionary returns an iterator that will loop +over the dictionary's keys:: + + >>> m = {'Jan': 1, 'Feb': 2, 'Mar': 3, 'Apr': 4, 'May': 5, 'Jun': 6, + ... 'Jul': 7, 'Aug': 8, 'Sep': 9, 'Oct': 10, 'Nov': 11, 'Dec': 12} + >>> for key in m: + ... print key, m[key] + Mar 3 + Feb 2 + Aug 8 + Sep 9 + May 5 + Jun 6 + Jul 7 + Jan 1 + Apr 4 + Nov 11 + Dec 12 + Oct 10 + +Note that the order is essentially random, because it's based on the +hash ordering of the objects in the dictionary. + +Applying ``iter()`` to a dictionary always loops over the keys, but +dictionaries have methods that return other iterators. If you want to +iterate over keys, values, or key/value pairs, you can explicitly call +the ``iterkeys()``, ``itervalues()``, or ``iteritems()`` methods to +get an appropriate iterator. + +The ``dict()`` constructor can accept an iterator that returns a +finite stream of ``(key, value)`` tuples:: + + >>> L = [('Italy', 'Rome'), ('France', 'Paris'), ('US', 'Washington DC')] + >>> dict(iter(L)) + {'Italy': 'Rome', 'US': 'Washington DC', 'France': 'Paris'} + +Files also support iteration by calling the ``readline()`` +method until there are no more lines in the file. This means you can +read each line of a file like this:: + + for line in file: + # do something for each line + ... + +Sets can take their contents from an iterable and let you iterate over +the set's elements:: + + S = set((2, 3, 5, 7, 11, 13)) + for i in S: + print i + + + +Generator expressions and list comprehensions +---------------------------------------------------- + +Two common operations on a stream are 1) performing some operation for +every element, 2) selecting a subset of elements that meet some +condition. For example, given a list of strings, you might want to +strip off trailing whitespace from each line or extract all the +strings containing a given substring. + +List comprehensions and generator expressions (short form: "listcomps" +and "genexps") are a concise notation for such operations, borrowed +from the functional programming language Haskell +(http://www.haskell.org). You can strip all the whitespace from a +stream of strings with the following code:: + + line_list = [' line 1\n', 'line 2 \n', ...] + + # Generator expression -- returns iterator + stripped_iter = (line.strip() for line in line_list) + + # List comprehension -- returns list + stripped_list = [line.strip() for line in line_list] + +You can select only certain elements by adding an ``"if"`` condition:: + + stripped_list = [line.strip() for line in line_list + if line != ""] + +With a list comprehension, you get back a Python list; +``stripped_list`` is a list containing the resulting lines, not an +iterator. Generator expressions return an iterator that computes the +values as necessary, not needing to materialize all the values at +once. This means that list comprehensions aren't useful if you're +working with iterators that return an infinite stream or a very large +amount of data. Generator expressions are preferable in these +situations. + +Generator expressions are surrounded by parentheses ("()") and list +comprehensions are surrounded by square brackets ("[]"). Generator +expressions have the form:: + + ( expression for expr in sequence1 + if condition1 + for expr2 in sequence2 + if condition2 + for expr3 in sequence3 ... + if condition3 + for exprN in sequenceN + if conditionN ) + +Again, for a list comprehension only the outside brackets are +different (square brackets instead of parentheses). + +The elements of the generated output will be the successive values of +``expression``. The ``if`` clauses are all optional; if present, +``expression`` is only evaluated and added to the result when +``condition`` is true. + +Generator expressions always have to be written inside parentheses, +but the parentheses signalling a function call also count. If you +want to create an iterator that will be immediately passed to a +function you can write:: + + obj_total = sum(obj.count for obj in list_all_objects()) + +The ``for...in`` clauses contain the sequences to be iterated over. +The sequences do not have to be the same length, because they are +iterated over from left to right, **not** in parallel. For each +element in ``sequence1``, ``sequence2`` is looped over from the +beginning. ``sequence3`` is then looped over for each +resulting pair of elements from ``sequence1`` and ``sequence2``. + +To put it another way, a list comprehension or generator expression is +equivalent to the following Python code:: + + for expr1 in sequence1: + if not (condition1): + continue # Skip this element + for expr2 in sequence2: + if not (condition2): + continue # Skip this element + ... + for exprN in sequenceN: + if not (conditionN): + continue # Skip this element + + # Output the value of + # the expression. + +This means that when there are multiple ``for...in`` clauses but no +``if`` clauses, the length of the resulting output will be equal to +the product of the lengths of all the sequences. If you have two +lists of length 3, the output list is 9 elements long:: + + seq1 = 'abc' + seq2 = (1,2,3) + >>> [ (x,y) for x in seq1 for y in seq2] + [('a', 1), ('a', 2), ('a', 3), + ('b', 1), ('b', 2), ('b', 3), + ('c', 1), ('c', 2), ('c', 3)] + +To avoid introducing an ambiguity into Python's grammar, if +``expression`` is creating a tuple, it must be surrounded with +parentheses. The first list comprehension below is a syntax error, +while the second one is correct:: + + # Syntax error + [ x,y for x in seq1 for y in seq2] + # Correct + [ (x,y) for x in seq1 for y in seq2] + + +Generators +----------------------- + +Generators are a special class of functions that simplify the task of +writing iterators. Regular functions compute a value and return it, +but generators return an iterator that returns a stream of values. + +You're doubtless familiar with how regular function calls work in +Python or C. When you call a function, it gets a private namespace +where its local variables are created. When the function reaches a +``return`` statement, the local variables are destroyed and the +value is returned to the caller. A later call to the same function +creates a new private namespace and a fresh set of local +variables. But, what if the local variables weren't thrown away on +exiting a function? What if you could later resume the function where +it left off? This is what generators provide; they can be thought of +as resumable functions. + +Here's the simplest example of a generator function:: + + def generate_ints(N): + for i in range(N): + yield i + +Any function containing a ``yield`` keyword is a generator function; +this is detected by Python's bytecode compiler which compiles the +function specially as a result. + +When you call a generator function, it doesn't return a single value; +instead it returns a generator object that supports the iterator +protocol. On executing the ``yield`` expression, the generator +outputs the value of ``i``, similar to a ``return`` +statement. The big difference between ``yield`` and a +``return`` statement is that on reaching a ``yield`` the +generator's state of execution is suspended and local variables are +preserved. On the next call to the generator's ``.next()`` method, +the function will resume executing. + +Here's a sample usage of the ``generate_ints()`` generator:: + + >>> gen = generate_ints(3) + >>> gen + <generator object at 0x8117f90> + >>> gen.next() + 0 + >>> gen.next() + 1 + >>> gen.next() + 2 + >>> gen.next() + Traceback (most recent call last): + File "stdin", line 1, in ? + File "stdin", line 2, in generate_ints + StopIteration + +You could equally write ``for i in generate_ints(5)``, or +``a,b,c = generate_ints(3)``. + +Inside a generator function, the ``return`` statement can only be used +without a value, and signals the end of the procession of values; +after executing a ``return`` the generator cannot return any further +values. ``return`` with a value, such as ``return 5``, is a syntax +error inside a generator function. The end of the generator's results +can also be indicated by raising ``StopIteration`` manually, or by +just letting the flow of execution fall off the bottom of the +function. + +You could achieve the effect of generators manually by writing your +own class and storing all the local variables of the generator as +instance variables. For example, returning a list of integers could +be done by setting ``self.count`` to 0, and having the +``next()`` method increment ``self.count`` and return it. +However, for a moderately complicated generator, writing a +corresponding class can be much messier. + +The test suite included with Python's library, ``test_generators.py``, +contains a number of more interesting examples. Here's one generator +that implements an in-order traversal of a tree using generators +recursively. + +:: + + # A recursive generator that generates Tree leaves in in-order. + def inorder(t): + if t: + for x in inorder(t.left): + yield x + + yield t.label + + for x in inorder(t.right): + yield x + +Two other examples in ``test_generators.py`` produce +solutions for the N-Queens problem (placing N queens on an NxN +chess board so that no queen threatens another) and the Knight's Tour +(finding a route that takes a knight to every square of an NxN chessboard +without visiting any square twice). + + + +Passing values into a generator +'''''''''''''''''''''''''''''''''''''''''''''' + +In Python 2.4 and earlier, generators only produced output. Once a +generator's code was invoked to create an iterator, there was no way to +pass any new information into the function when its execution is +resumed. You could hack together this ability by making the +generator look at a global variable or by passing in some mutable object +that callers then modify, but these approaches are messy. + +In Python 2.5 there's a simple way to pass values into a generator. +``yield`` became an expression, returning a value that can be assigned +to a variable or otherwise operated on:: + + val = (yield i) + +I recommend that you **always** put parentheses around a ``yield`` +expression when you're doing something with the returned value, as in +the above example. The parentheses aren't always necessary, but it's +easier to always add them instead of having to remember when they're +needed. + +(PEP 342 explains the exact rules, which are that a +``yield``-expression must always be parenthesized except when it +occurs at the top-level expression on the right-hand side of an +assignment. This means you can write ``val = yield i`` but have to +use parentheses when there's an operation, as in ``val = (yield i) ++ 12``.) + +Values are sent into a generator by calling its +``send(value)`` method. This method resumes the +generator's code and the ``yield`` expression returns the specified +value. If the regular ``next()`` method is called, the +``yield`` returns ``None``. + +Here's a simple counter that increments by 1 and allows changing the +value of the internal counter. + +:: + + def counter (maximum): + i = 0 + while i < maximum: + val = (yield i) + # If value provided, change counter + if val is not None: + i = val + else: + i += 1 + +And here's an example of changing the counter: + + >>> it = counter(10) + >>> print it.next() + 0 + >>> print it.next() + 1 + >>> print it.send(8) + 8 + >>> print it.next() + 9 + >>> print it.next() + Traceback (most recent call last): + File ``t.py'', line 15, in ? + print it.next() + StopIteration + +Because ``yield`` will often be returning ``None``, you +should always check for this case. Don't just use its value in +expressions unless you're sure that the ``send()`` method +will be the only method used resume your generator function. + +In addition to ``send()``, there are two other new methods on +generators: + +* ``throw(type, value=None, traceback=None)`` is used to raise an exception inside the + generator; the exception is raised by the ``yield`` expression + where the generator's execution is paused. + +* ``close()`` raises a ``GeneratorExit`` + exception inside the generator to terminate the iteration. + On receiving this + exception, the generator's code must either raise + ``GeneratorExit`` or ``StopIteration``; catching the + exception and doing anything else is illegal and will trigger + a ``RuntimeError``. ``close()`` will also be called by + Python's garbage collector when the generator is garbage-collected. + + If you need to run cleanup code when a ``GeneratorExit`` occurs, + I suggest using a ``try: ... finally:`` suite instead of + catching ``GeneratorExit``. + +The cumulative effect of these changes is to turn generators from +one-way producers of information into both producers and consumers. + +Generators also become **coroutines**, a more generalized form of +subroutines. Subroutines are entered at one point and exited at +another point (the top of the function, and a ``return`` +statement), but coroutines can be entered, exited, and resumed at +many different points (the ``yield`` statements). + + +Built-in functions +---------------------------------------------- + +Let's look in more detail at built-in functions often used with iterators. + +Two Python's built-in functions, ``map()`` and ``filter()``, are +somewhat obsolete; they duplicate the features of list comprehensions +and return actual lists instead of iterators. + +``map(f, iterA, iterB, ...)`` returns a list containing ``f(iterA[0], +iterB[0]), f(iterA[1], iterB[1]), f(iterA[2], iterB[2]), ...``. + +:: + + def upper(s): + return s.upper() + map(upper, ['sentence', 'fragment']) => + ['SENTENCE', 'FRAGMENT'] + + [upper(s) for s in ['sentence', 'fragment']] => + ['SENTENCE', 'FRAGMENT'] + +As shown above, you can achieve the same effect with a list +comprehension. The ``itertools.imap()`` function does the same thing +but can handle infinite iterators; it'll be discussed in the section on +the ``itertools`` module. + +``filter(predicate, iter)`` returns a list +that contains all the sequence elements that meet a certain condition, +and is similarly duplicated by list comprehensions. +A **predicate** is a function that returns the truth value of +some condition; for use with ``filter()``, the predicate must take a +single value. + +:: + + def is_even(x): + return (x % 2) == 0 + + filter(is_even, range(10)) => + [0, 2, 4, 6, 8] + +This can also be written as a list comprehension:: + + >>> [x for x in range(10) if is_even(x)] + [0, 2, 4, 6, 8] + +``filter()`` also has a counterpart in the ``itertools`` module, +``itertools.ifilter()``, that returns an iterator and +can therefore handle infinite sequences just as ``itertools.imap()`` can. + +``reduce(func, iter, [initial_value])`` doesn't have a counterpart in +the ``itertools`` module because it cumulatively performs an operation +on all the iterable's elements and therefore can't be applied to +infinite ones. ``func`` must be a function that takes two elements +and returns a single value. ``reduce()`` takes the first two elements +A and B returned by the iterator and calculates ``func(A, B)``. It +then requests the third element, C, calculates ``func(func(A, B), +C)``, combines this result with the fourth element returned, and +continues until the iterable is exhausted. If the iterable returns no +values at all, a ``TypeError`` exception is raised. If the initial +value is supplied, it's used as a starting point and +``func(initial_value, A)`` is the first calculation. + +:: + + import operator + reduce(operator.concat, ['A', 'BB', 'C']) => + 'ABBC' + reduce(operator.concat, []) => + TypeError: reduce() of empty sequence with no initial value + reduce(operator.mul, [1,2,3], 1) => + 6 + reduce(operator.mul, [], 1) => + 1 + +If you use ``operator.add`` with ``reduce()``, you'll add up all the +elements of the iterable. This case is so common that there's a special +built-in called ``sum()`` to compute it:: + + reduce(operator.add, [1,2,3,4], 0) => + 10 + sum([1,2,3,4]) => + 10 + sum([]) => + 0 + +For many uses of ``reduce()``, though, it can be clearer to just write +the obvious ``for`` loop:: + + # Instead of: + product = reduce(operator.mul, [1,2,3], 1) + + # You can write: + product = 1 + for i in [1,2,3]: + product *= i + + +``enumerate(iter)`` counts off the elements in the iterable, returning +2-tuples containing the count and each element. + +:: + + enumerate(['subject', 'verb', 'object']) => + (0, 'subject'), (1, 'verb'), (2, 'object') + +``enumerate()`` is often used when looping through a list +and recording the indexes at which certain conditions are met:: + + f = open('data.txt', 'r') + for i, line in enumerate(f): + if line.strip() == '': + print 'Blank line at line #%i' % i + +``sorted(iterable, [cmp=None], [key=None], [reverse=False)`` +collects all the elements of the iterable into a list, sorts +the list, and returns the sorted result. The ``cmp``, ``key``, +and ``reverse`` arguments are passed through to the +constructed list's ``.sort()`` method. + +:: + + import random + # Generate 8 random numbers between [0, 10000) + rand_list = random.sample(range(10000), 8) + rand_list => + [769, 7953, 9828, 6431, 8442, 9878, 6213, 2207] + sorted(rand_list) => + [769, 2207, 6213, 6431, 7953, 8442, 9828, 9878] + sorted(rand_list, reverse=True) => + [9878, 9828, 8442, 7953, 6431, 6213, 2207, 769] + +(For a more detailed discussion of sorting, see the Sorting mini-HOWTO +in the Python wiki at http://wiki.python.org/moin/HowTo/Sorting.) + +The ``any(iter)`` and ``all(iter)`` built-ins look at +the truth values of an iterable's contents. ``any()`` returns +True if any element in the iterable is a true value, and ``all()`` +returns True if all of the elements are true values:: + + any([0,1,0]) => + True + any([0,0,0]) => + False + any([1,1,1]) => + True + all([0,1,0]) => + False + all([0,0,0]) => + False + all([1,1,1]) => + True + + +Small functions and the lambda statement +---------------------------------------------- + +When writing functional-style programs, you'll often need little +functions that act as predicates or that combine elements in some way. + +If there's a Python built-in or a module function that's suitable, you +don't need to define a new function at all:: + + stripped_lines = [line.strip() for line in lines] + existing_files = filter(os.path.exists, file_list) + +If the function you need doesn't exist, you need to write it. One way +to write small functions is to use the ``lambda`` statement. ``lambda`` +takes a number of parameters and an expression combining these parameters, +and creates a small function that returns the value of the expression: + + lowercase = lambda x: x.lower() + + print_assign = lambda name, value: name + '=' + str(value) + + adder = lambda x, y: x+y + +An alternative is to just use the ``def`` statement and define a +function in the usual way:: + + def lowercase(x): + return x.lower() + + def print_assign(name, value): + return name + '=' + str(value) + + def adder(x,y): + return x + y + +Which alternative is preferable? That's a style question; my usual +view is to avoid using ``lambda``. + +``lambda`` is quite limited in the functions it can define. The +result has to be computable as a single expression, which means you +can't have multiway ``if... elif... else`` comparisons or +``try... except`` statements. If you try to do too much in a +``lambda`` statement, you'll end up with an overly complicated +expression that's hard to read. Quick, what's the following code doing? + +:: + + total = reduce(lambda a, b: (0, a[1] + b[1]), items)[1] + +You can figure it out, but it takes time to disentangle the expression +to figure out what's going on. Using a short nested +``def`` statements makes things a little bit better:: + + def combine (a, b): + return 0, a[1] + b[1] + + total = reduce(combine, items)[1] + +But it would be best of all if I had simply used a ``for`` loop:: + + total = 0 + for a, b in items: + total += b + +Or the ``sum()`` built-in and a generator expression:: + + total = sum(b for a,b in items) + +Many uses of ``reduce()`` are clearer when written as ``for`` loops. + +Fredrik Lundh once suggested the following set of rules for refactoring +uses of ``lambda``: + +1) Write a lambda function. +2) Write a comment explaining what the heck that lambda does. +3) Study the comment for a while, and think of a name that captures + the essence of the comment. +4) Convert the lambda to a def statement, using that name. +5) Remove the comment. + +I really like these rules, but you're free to disagree that this style +is better. + + +The itertools module +----------------------- + +The ``itertools`` module contains a number of commonly-used iterators +as well as functions for combining several iterators. This section +will introduce the module's contents by showing small examples. + +``itertools.count(n)`` returns an infinite stream of +integers, increasing by 1 each time. You can optionally supply the +starting number, which defaults to 0:: + + itertools.count() => + 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, ... + itertools.count(10) => + 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, ... + +``itertools.cycle(iter)`` saves a copy of the contents of a provided +iterable and returns a new iterator that returns its elements from +first to last. The new iterator will repeat these elements infinitely. + +:: + + itertools.cycle([1,2,3,4,5]) => + 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, ... + +``itertools.repeat(elem, [n])`` returns the provided element ``n`` +times, or returns the element endlessly if ``n`` is not provided. + +:: + + itertools.repeat('abc') => + abc, abc, abc, abc, abc, abc, abc, abc, abc, abc, ... + itertools.repeat('abc', 5) => + abc, abc, abc, abc, abc + +``itertools.chain(iterA, iterB, ...)`` takes an arbitrary number of +iterables as input, and returns all the elements of the first +iterator, then all the elements of the second, and so on, until all of +the iterables have been exhausted. + +:: + + itertools.chain(['a', 'b', 'c'], (1, 2, 3)) => + a, b, c, 1, 2, 3 + +``itertools.izip(iterA, iterB, ...)`` takes one element from each iterable +and returns them in a tuple:: + + itertools.izip(['a', 'b', 'c'], (1, 2, 3)) => + ('a', 1), ('b', 2), ('c', 3) + +This iterator is intended to be used with iterables that are all of +the same length. If the iterables are of different lengths, the +resulting stream will be the same length as the shortest iterable. + +:: + + itertools.izip(['a', 'b'], (1, 2, 3)) => + ('a', 1), ('b', 2) + +You should avoid doing this, though, because an element may be taken +from the longer iterators and discarded. This means you can't go on +to use the iterators further because you risk skipping a discarded +element. + +``itertools.islice(iter, [start], stop, [step])`` returns a stream +that's a slice of the iterator. It can return the first ``stop`` +elements. If you supply a starting index, you'll get ``stop-start`` +elements, and if you supply a value for ``step` elements will be +skipped accordingly. Unlike Python's string and list slicing, you +can't use negative values for ``start``, ``stop``, or ``step``. + +:: + + itertools.islice(range(10), 8) => + 0, 1, 2, 3, 4, 5, 6, 7 + itertools.islice(range(10), 2, 8) => + 2, 3, 4, 5, 6, 7 + itertools.islice(range(10), 2, 8, 2) => + 2, 4, 6 + +``itertools.tee(iter, [n])`` replicates an iterator; it returns ``n`` +independent iterators that will all return the contents of the source +iterator. If you don't supply a value for ``n``, the default is 2. +Replicating iterators requires saving some of the contents of the source +iterator, so this can consume significant memory if the iterator is large +and one of the new iterators is consumed more than the others. + +:: + + itertools.tee( itertools.count() ) => + iterA, iterB + + where iterA -> + 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, ... + + and iterB -> + 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, ... + + +Two functions are used for calling other functions on the contents of an +iterable. + +``itertools.imap(f, iterA, iterB, ...)`` returns +a stream containing ``f(iterA[0], iterB[0]), f(iterA[1], iterB[1]), +f(iterA[2], iterB[2]), ...``:: + + itertools.imap(operator.add, [5, 6, 5], [1, 2, 3]) => + 6, 8, 8 + +The ``operator`` module contains a set of functions +corresponding to Python's operators. Some examples are +``operator.add(a, b)`` (adds two values), +``operator.ne(a, b)`` (same as ``a!=b``), +and +``operator.attrgetter('id')`` (returns a callable that +fetches the ``"id"`` attribute). + +``itertools.starmap(func, iter)`` assumes that the iterable will +return a stream of tuples, and calls ``f()`` using these tuples as the +arguments:: + + itertools.starmap(os.path.join, + [('/usr', 'bin', 'java'), ('/bin', 'python'), + ('/usr', 'bin', 'perl'),('/usr', 'bin', 'ruby')]) + => + /usr/bin/java, /bin/python, /usr/bin/perl, /usr/bin/ruby + +Another group of functions chooses a subset of an iterator's elements +based on a predicate. + +``itertools.ifilter(predicate, iter)`` returns all the elements for +which the predicate returns true:: + + def is_even(x): + return (x % 2) == 0 + + itertools.ifilter(is_even, itertools.count()) => + 0, 2, 4, 6, 8, 10, 12, 14, ... + +``itertools.ifilterfalse(predicate, iter)`` is the opposite, +returning all elements for which the predicate returns false:: + + itertools.ifilterfalse(is_even, itertools.count()) => + 1, 3, 5, 7, 9, 11, 13, 15, ... + +``itertools.takewhile(predicate, iter)`` returns elements for as long +as the predicate returns true. Once the predicate returns false, +the iterator will signal the end of its results. + +:: + + def less_than_10(x): + return (x < 10) + + itertools.takewhile(less_than_10, itertools.count()) => + 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 + + itertools.takewhile(is_even, itertools.count()) => + 0 + +``itertools.dropwhile(predicate, iter)`` discards elements while the +predicate returns true, and then returns the rest of the iterable's +results. + +:: + + itertools.dropwhile(less_than_10, itertools.count()) => + 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, ... + + itertools.dropwhile(is_even, itertools.count()) => + 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ... + + +The last function I'll discuss, ``itertools.groupby(iter, +key_func=None)``, is the most complicated. ``key_func(elem)`` is a +function that can compute a key value for each element returned by the +iterable. If you don't supply a key function, the key is simply each +element itself. + +``groupby()`` collects all the consecutive elements from the +underlying iterable that have the same key value, and returns a stream +of 2-tuples containing a key value and an iterator for the elements +with that key. + +:: + + city_list = [('Decatur', 'AL'), ('Huntsville', 'AL'), ('Selma', 'AL'), + ('Anchorage', 'AK'), ('Nome', 'AK'), + ('Flagstaff', 'AZ'), ('Phoenix', 'AZ'), ('Tucson', 'AZ'), + ... + ] + + def get_state ((city, state)): + return state + + itertools.groupby(city_list, get_state) => + ('AL', iterator-1), + ('AK', iterator-2), + ('AZ', iterator-3), ... + + where + iterator-1 => + ('Decatur', 'AL'), ('Huntsville', 'AL'), ('Selma', 'AL') + iterator-2 => + ('Anchorage', 'AK'), ('Nome', 'AK') + iterator-3 => + ('Flagstaff', 'AZ'), ('Phoenix', 'AZ'), ('Tucson', 'AZ') + +``groupby()`` assumes that the underlying iterable's contents will +already be sorted based on the key. Note that the returned iterators +also use the underlying iterable, so you have to consume the results +of iterator-1 before requesting iterator-2 and its corresponding key. + + +The functools module +---------------------------------------------- + +The ``functools`` module in Python 2.5 contains some higher-order +functions. A **higher-order function** takes functions as input and +returns new functions. The most useful tool in this module is the +``partial()`` function. + +For programs written in a functional style, you'll sometimes want to +construct variants of existing functions that have some of the +parameters filled in. Consider a Python function ``f(a, b, c)``; you +may wish to create a new function ``g(b, c)`` that was equivalent to +``f(1, b, c)``. This is called "partial function application". + +The constructor for ``partial`` takes the arguments ``(function, arg1, +arg2, ... kwarg1=value1, kwarg2=value2)``. The resulting object is +callable, so you can just call it to invoke ``function`` with the +filled-in arguments. + +Here's a small but realistic example:: + + import functools + + def log (message, subsystem): + "Write the contents of 'message' to the specified subsystem." + print '%s: %s' % (subsystem, message) + ... + + server_log = functools.partial(log, subsystem='server') + server_log('Unable to open socket') + +There are also third-party modules, such as Collin Winter's +`functional package <http://cheeseshop.python.org/pypi/functional>`__, +that are intended for use in functional-style programs. + + +Revision History and Acknowledgements +------------------------------------------------ + +The author would like to thank the following people for offering +suggestions, corrections and assistance with various drafts of this +article: Ian Bicking, Nick Coghlan, Nick Efford, Raymond Hettinger, +Jim Jewett, Mike Krell, Leandro Lameiro, Jussi Salmela, +Collin Winter, Blake Winton. + +Version 0.1: posted June 30 2006. + +Version 0.11: posted July 1 2006. Typo fixes. + +Version 0.2: posted July 10 2006. Merged genexp and listcomp +sections into one. Typo fixes. + +Version 0.21: Added more references suggested on the tutor mailing list. + + +References +-------------------- + +General +''''''''''''''' + +**Structure and Interpretation of Computer Programs**, by +Harold Abelson and Gerald Jay Sussman with Julie Sussman. +Full text at http://mitpress.mit.edu/sicp/. +In this classic textbook of computer science, chapters 2 and 3 discuss the +use of sequences and streams to organize the data flow inside a +program. The book uses Scheme for its examples, but many of the +design approaches described in these chapters are applicable to +functional-style Python code. + +http://www.defmacro.org/ramblings/fp.html: A general +introduction to functional programming that uses Java examples +and has a lengthy historical introduction. + +http://en.wikipedia.org/wiki/Functional_programming: +General Wikipedia entry describing functional programming. + +http://en.wikipedia.org/wiki/Coroutine: +Entry for coroutines. + + +Python-specific +''''''''''''''''''''''''''' + +http://gnosis.cx/TPiP/: +The first chapter of David Mertz's book :title-reference:`Text Processing in Python` +discusses functional programming for text processing, in the section titled +"Utilizing Higher-Order Functions in Text Processing". + +Mertz also wrote a 3-part series of articles on functional programming +for IBM's DeveloperWorks site; see +`part 1 <http://www-128.ibm.com/developerworks/library/l-prog.html>`__, +`part 2 <http://www-128.ibm.com/developerworks/library/l-prog2.html>`__, and +`part 3 <http://www-128.ibm.com/developerworks/linux/library/l-prog3.html>`__, + + +Python documentation +''''''''''''''''''''''''''' + +http://docs.python.org/lib/module-itertools.html: +Documentation ``for the itertools`` module. + +http://docs.python.org/lib/module-operator.html: +Documentation ``for the operator`` module. + +http://www.python.org/dev/peps/pep-0289/: +PEP 289: "Generator Expressions" + +http://www.python.org/dev/peps/pep-0342/ +PEP 342: "Coroutines via Enhanced Generators" describes the new generator +features in Python 2.5. + +.. comment + + Topics to place + ----------------------------- + + XXX os.walk() + + XXX Need a large example. + + But will an example add much? I'll post a first draft and see + what the comments say. + +.. comment + + Original outline: + Introduction + Idea of FP + Programs built out of functions + Functions are strictly input-output, no internal state + Opposed to OO programming, where objects have state + + Why FP? + Formal provability + Assignment is difficult to reason about + Not very relevant to Python + Modularity + Small functions that do one thing + Debuggability: + Easy to test due to lack of state + Easy to verify output from intermediate steps + Composability + You assemble a toolbox of functions that can be mixed + + Tackling a problem + Need a significant example + + Iterators + Generators + The itertools module + List comprehensions + Small functions and the lambda statement + Built-in functions + map + filter + reduce + +.. comment + + Handy little function for printing part of an iterator -- used + while writing this document. + + import itertools + def print_iter(it): + slice = itertools.islice(it, 10) + for elem in slice[:-1]: + sys.stdout.write(str(elem)) + sys.stdout.write(', ') + print elem[-1] + + |