diff options
| -rw-r--r-- | Lib/_collections_abc.py | 26 | ||||
| -rw-r--r-- | Lib/numbers.py | 25 |
2 files changed, 51 insertions, 0 deletions
diff --git a/Lib/_collections_abc.py b/Lib/_collections_abc.py index e96e4c3535..c62233b81a 100644 --- a/Lib/_collections_abc.py +++ b/Lib/_collections_abc.py @@ -6,6 +6,32 @@ Unit tests are in test_collections. """ +############ Maintenance notes ######################################### +# +# ABCs are different from other standard library modules in that they +# specify compliance tests. In general, once an ABC has been published, +# new methods (either abstract or concrete) cannot be added. +# +# Though classes that inherit from an ABC would automatically receive a +# new mixin method, registered classes would become non-compliant and +# violate the contract promised by ``isinstance(someobj, SomeABC)``. +# +# Though irritating, the correct procedure for adding new abstract or +# mixin methods is to create a new ABC as a subclass of the previous +# ABC. For example, union(), intersection(), and difference() cannot +# be added to Set but could go into a new ABC that extends Set. +# +# Because they are so hard to change, new ABCs should have their APIs +# carefully thought through prior to publication. +# +# Since ABCMeta only checks for the presence of methods, it is possible +# to alter the signature of a method by adding optional arguments +# or changing parameters names. This is still a bit dubious but at +# least it won't cause isinstance() to return an incorrect result. +# +# +####################################################################### + from abc import ABCMeta, abstractmethod import sys diff --git a/Lib/numbers.py b/Lib/numbers.py index 5b98e64208..9548aebd77 100644 --- a/Lib/numbers.py +++ b/Lib/numbers.py @@ -5,6 +5,31 @@ TODO: Fill out more detailed documentation on the operators.""" +############ Maintenance notes ######################################### +# +# ABCs are different from other standard library modules in that they +# specify compliance tests. In general, once an ABC has been published, +# new methods (either abstract or concrete) cannot be added. +# +# Though classes that inherit from an ABC would automatically receive a +# new mixin method, registered classes would become non-compliant and +# violate the contract promised by ``isinstance(someobj, SomeABC)``. +# +# Though irritating, the correct procedure for adding new abstract or +# mixin methods is to create a new ABC as a subclass of the previous +# ABC. +# +# Because they are so hard to change, new ABCs should have their APIs +# carefully thought through prior to publication. +# +# Since ABCMeta only checks for the presence of methods, it is possible +# to alter the signature of a method by adding optional arguments +# or changing parameter names. This is still a bit dubious but at +# least it won't cause isinstance() to return an incorrect result. +# +# +####################################################################### + from abc import ABCMeta, abstractmethod __all__ = ["Number", "Complex", "Real", "Rational", "Integral"] |