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# Copyright (c) 2010-2012 testtools developers. See LICENSE for details.
import sys
def try_import(name, alternative=None, error_callback=None):
"""Attempt to import a module, with a fallback.
Attempt to import ``name``. If it fails, return ``alternative``. When
supporting multiple versions of Python or optional dependencies, it is
useful to be able to try to import a module.
:param name: The name of the object to import, e.g. ``os.path`` or
``os.path.join``.
:param alternative: The value to return if no module can be imported.
Defaults to None.
:param error_callback: If non-None, a callable that is passed the
ImportError when the module cannot be loaded.
"""
module_segments = name.split('.')
last_error = None
remainder = []
# module_name will be what successfully imports. We cannot walk from the
# __import__ result because in import loops (A imports A.B, which imports
# C, which calls try_import("A.B")) A.B will not yet be set.
while module_segments:
module_name = '.'.join(module_segments)
try:
__import__(module_name)
except ImportError:
last_error = sys.exc_info()[1]
remainder.append(module_segments.pop())
continue
else:
break
else:
if last_error is not None and error_callback is not None:
error_callback(last_error)
return alternative
module = sys.modules[module_name]
nonexistent = object()
for segment in reversed(remainder):
module = getattr(module, segment, nonexistent)
if module is nonexistent:
if last_error is not None and error_callback is not None:
error_callback(last_error)
return alternative
return module
def map_values(function, dictionary):
"""Map ``function`` across the values of ``dictionary``.
:return: A dict with the same keys as ``dictionary``, where the value
of each key ``k`` is ``function(dictionary[k])``.
"""
return {k: function(dictionary[k]) for k in dictionary}
def filter_values(function, dictionary):
"""Filter ``dictionary`` by its values using ``function``."""
return {k: v for k, v in dictionary.items() if function(v)}
def dict_subtract(a, b):
"""Return the part of ``a`` that's not in ``b``."""
return {k: a[k] for k in set(a) - set(b)}
def list_subtract(a, b):
"""Return a list ``a`` without the elements of ``b``.
If a particular value is in ``a`` twice and ``b`` once then the returned
list then that value will appear once in the returned list.
"""
a_only = list(a)
for x in b:
if x in a_only:
a_only.remove(x)
return a_only
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