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copied!<p>This is what I justed in my unittests.</p> <p>It uses <a href="http://docs.python.org/release/2.3.5/whatsnew/section-pep302.html" rel="nofollow noreferrer">PEP-302 "New Import Hooks"</a>. (Warning: the PEP-302 document and the more concise release notes I linked aren't exactly <em>accurate</em>.)</p> <p>I use <code>meta_path</code> because it's as early as possible in the import sequence.</p> <p>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 <code>reload</code> on the dependent module.</p> <pre><code>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) </code></pre> <p>Where the code inside pif.index looks like:</p> <pre><code>try: import xdg.BaseDirectory CONFIG_DIR = os.path.join(xdg.BaseDirectory.xdg_data_home, 'pif') except ImportError: CONFIG_DIR = os.path.expanduser('~/.pif') </code></pre> <hr> <p>To answer the question about why the newly reloaded module has properties of the old and new loads, here are two example files.</p> <p>The first is a module <code>y</code> with an import failure case.</p> <pre><code># y.py try: import sys _loaded_with = 'sys' except ImportError: import os _loaded_with = 'os' </code></pre> <p>The second is <code>x</code> which demonstrates how leaving handles about for a module can affect its properties when being reloaded.</p> <pre><code># 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 </code></pre>

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