Fixes #421 - Make all dependencies optional

Thanks Hugo van Kemenade for the review.

3 files changed, 71 insertions, 26 deletions

diff --git a/src/tablib/core.py b/src/tablib/core.py
index d17e1e8..9fcbebd 100644
--- a/src/tablib/core.py
+++ b/src/tablib/core.py
@@ -13,6 +13,13 @@ from copy import copy
 from operator import itemgetter
 
 from tablib import formats
+from tablib.exceptions import (
+    HeadersNeeded,
+    InvalidDatasetIndex,
+    InvalidDatasetType,
+    InvalidDimensions,
+    UnsupportedFormat,
+)
 from tablib.formats import registry
 
 __title__ = 'tablib'
@@ -903,24 +910,4 @@ def import_book(stream, format=None, **kwargs):
     return Databook().load(stream, format, **kwargs)
 
 
-class InvalidDatasetType(Exception):
-    "Only Datasets can be added to a DataBook"
-
-
-class InvalidDimensions(Exception):
-    "Invalid size"
-
-
-class InvalidDatasetIndex(Exception):
-    "Outside of Dataset size"
-
-
-class HeadersNeeded(Exception):
-    "Header parameter must be given when appending a column in this Dataset."
-
-
-class UnsupportedFormat(NotImplementedError):
-    "Format is not supported"
-
-
 registry.register_builtins()
diff --git a/src/tablib/exceptions.py b/src/tablib/exceptions.py
new file mode 100644
index 0000000..dee2b29
--- /dev/null
+++ b/src/tablib/exceptions.py
@@ -0,0 +1,18 @@
+class InvalidDatasetType(Exception):
+    "Only Datasets can be added to a DataBook"
+
+
+class InvalidDimensions(Exception):
+    "Invalid size"
+
+
+class InvalidDatasetIndex(Exception):
+    "Outside of Dataset size"
+
+
+class HeadersNeeded(Exception):
+    "Header parameter must be given when appending a column in this Dataset."
+
+
+class UnsupportedFormat(NotImplementedError):
+    "Format is not supported"
diff --git a/src/tablib/formats/__init__.py b/src/tablib/formats/__init__.py
index a462ce0..c324484 100644
--- a/src/tablib/formats/__init__.py
+++ b/src/tablib/formats/__init__.py
@@ -2,6 +2,9 @@
 """
 from collections import OrderedDict
 from functools import partialmethod
+from importlib.util import find_spec
+
+from tablib.exceptions import UnsupportedFormat
 
 from ._csv import CSVFormat
 from ._dbf import DBFFormat
@@ -17,6 +20,33 @@ from ._xls import XLSFormat
 from ._xlsx import XLSXFormat
 from ._yaml import YAMLFormat
 
+uninstalled_format_messages = {
+    'df': (
+        "The 'df' format is not available. You may want to install the pandas "
+        "package (or `pip install tablib[pandas]`)."
+    ),
+    'html': (
+        "The 'html' format is not available. You may want to install the MarkupPy "
+        "package (or `pip install tablib[html]`)."
+    ),
+    'ods': (
+        "The 'ods' format is not available. You may want to install the odfpy "
+        "package (or `pip install tablib[ods]`)."
+    ),
+    'xls': (
+        "The 'xls' format is not available. You may want to install the xlrd and "
+        "xlwt packages (or `pip install tablib[xls]`)."
+    ),
+    'xlsx': (
+        "The 'xlsx' format is not available. You may want to install the openpyxl "
+        "package (or `pip install tablib[xlsx]`)."
+    ),
+    'yaml': (
+        "The 'yaml' format is not available. You may want to install the pyyaml "
+        "package (or `pip install tablib[yaml]`)."
+    ),
+}
+
 
 class Registry:
     _formats = OrderedDict()
@@ -53,17 +83,23 @@ class Registry:
         # Registration ordering matters for autodetection.
         self.register('json', JSONFormat())
         # xlsx before as xls (xlrd) can also read xlsx
-        self.register('xlsx', XLSXFormat())
-        self.register('xls', XLSFormat())
-        self.register('yaml', YAMLFormat())
+        if find_spec('openpyxl'):
+            self.register('xlsx', XLSXFormat())
+        if find_spec('xlrd') and find_spec('xlwt'):
+            self.register('xls', XLSFormat())
+        if find_spec('yaml'):
+            self.register('yaml', YAMLFormat())
         self.register('csv', CSVFormat())
         self.register('tsv', TSVFormat())
-        self.register('ods', ODSFormat())
+        if find_spec('odf'):
+            self.register('ods', ODSFormat())
         self.register('dbf', DBFFormat())
-        self.register('html', HTMLFormat())
+        if find_spec('MarkupPy'):
+            self.register('html', HTMLFormat())
         self.register('jira', JIRAFormat())
         self.register('latex', LATEXFormat())
-        self.register('df', DataFrameFormat())
+        if find_spec('pandas'):
+            self.register('df', DataFrameFormat())
         self.register('rst', ReSTFormat())
 
     def formats(self):
@@ -71,6 +107,10 @@ class Registry:
             yield frm
 
     def get_format(self, key):
+        if key not in self._formats:
+            if key in uninstalled_format_messages:
+                raise UnsupportedFormat(uninstalled_format_messages[key])
+            raise UnsupportedFormat("Tablib has no format '%s' or it is not registered." % key)
         return self._formats[key]