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Mocking ImportError in Python

由 若如初见. 提交于 2019-11-27 23:12:49
Alex Martelli

Just monkeypatch into the builtins your own version of __import__ -- it can raise whatever you wish when it recognizes it's being called on the specific modules for which you want to mock up errors. See the docs for copious detail. Roughly:

try:
    import builtins
except ImportError:
    import __builtin__ as builtins
realimport = builtins.__import__

def myimport(name, globals, locals, fromlist, level):
    if ...:
        raise ImportError
    return realimport(name, globals, locals, fromlist, level)

builtins.__import__ = myimport

In lieu of the ..., you can hardcode name == 'zope.component', or arrange things more flexibly with a callback of your own that can make imports raise on demand in different cases, depending on your specific testing needs, without requiring you to code multiple __import__-alike functions;-).

Note also that if what you use, instead of import zope.component or from zope.component import something, is from zope import component, the name will then be 'zope', and 'component' will then be the only item in the fromlist.

Edit: the docs for the __import__ function say that the name to import is builtin (like in Python 3), but in fact you need __builtins__ -- I've edited the code above so that it works either way.

This is what I justed in my unittests.

It uses PEP-302 "New Import Hooks". (Warning: the PEP-302 document and the more concise release notes I linked aren't exactly accurate.)

I use meta_path because it's as early as possible in the import sequence.

If the module has already been imported (as in my case, because earlier unittests mock against it), then it's necessary to remove it from sys.modules before doing the reload on the dependent module.

Ensure we fallback to using ~/.pif if XDG doesn't exist.

 >>> import sys

 >>> class _():
 ... def __init__(self, modules):
 ...  self.modules = modules
 ...
 ...  def find_module(self, fullname, path=None):
 ...  if fullname in self.modules:
 ...   raise ImportError('Debug import failure for %s' % fullname)

 >>> fail_loader = _(['xdg.BaseDirectory'])
 >>> sys.meta_path.append(fail_loader)

 >>> del sys.modules['xdg.BaseDirectory']

 >>> reload(pif.index) #doctest: +ELLIPSIS
 <module 'pif.index' from '...'>

 >>> pif.index.CONFIG_DIR == os.path.expanduser('~/.pif')
 True

 >>> sys.meta_path.remove(fail_loader)

Where the code inside pif.index looks like:

try:
    import xdg.BaseDirectory

    CONFIG_DIR = os.path.join(xdg.BaseDirectory.xdg_data_home, 'pif')
except ImportError:
    CONFIG_DIR = os.path.expanduser('~/.pif')

To answer the question about why the newly reloaded module has properties of the old and new loads, here are two example files.

The first is a module y with an import failure case.

# y.py

try:
    import sys

    _loaded_with = 'sys'
except ImportError:
    import os

    _loaded_with = 'os'

The second is x which demonstrates how leaving handles about for a module can affect its properties when being reloaded.

# x.py

import sys

import y

assert y._loaded_with == 'sys'
assert y.sys

class _():
    def __init__(self, modules):
        self.modules = modules

    def find_module(self, fullname, path=None):
        if fullname in self.modules:
            raise ImportError('Debug import failure for %s' % fullname)

# Importing sys will not raise an ImportError.
fail_loader = _(['sys'])
sys.meta_path.append(fail_loader)

# Demonstrate that reloading doesn't work if the module is already in the
# cache.

reload(y)

assert y._loaded_with == 'sys'
assert y.sys

# Now we remove sys from the modules cache, and try again.
del sys.modules['sys']

reload(y)

assert y._loaded_with == 'os'
assert y.sys
assert y.os

# Now we remove the handles to the old y so it can get garbage-collected.
del sys.modules['y']
del y

import y

assert y._loaded_with == 'os'
try:
    assert y.sys
except AttributeError:
    pass
assert y.os

If you don't mind changing your program itself, you could also put the import call in a function and patch that in your tests.

标签
python
unit-testing
mocking
doctest
zope.component
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