什么会导致python模块被导入两次？

Question

据我所知，python模块永远不会导入两次，也就是说，模块中的代码只会在第一次导入时执行。后续的导入语句只是将模块添加到导入的作用域中。

我有一个名为"TiledConvC3D.py“的模块，它似乎被多次导入。我使用pdb打印此模块代码顶部的堆栈。

下面是第一次执行模块时堆栈跟踪的结尾：

File "<anonymized>/python_modules/Theano/theano/gof/cmodule.py", line 328, in refresh
  key = cPickle.load(open(key_pkl, 'rb'))
File "<anonymized>/ops/TiledConvG3D.py", line 565, in <module>
  import TiledConvC3D
File "<anonymized>/ops/TiledConvC3D.py", line 18, in <module>
  pdb.traceback.print_stack()

它还会被执行几次。但是，第二次调用它时，完整的堆栈跟踪不会显示任何对reload的调用，因此这些执行应该不会发生：

File "sup_train_conj_grad.py", line 103, in <module>
  dataset = Config.get_dataset(dataset_node)
File "<anonymized>/Config.py", line 279, in get_dataset
  from datasets import NewWiskott
File "<anonymized>/datasets/NewWiskott.py", line 16, in <module>
  normalizer_train = video.ContrastNormalizer3D(sigma, global_per_frame = False, input_is_5d = True)
File "<anonymized>/util/video.py", line 204, in __init__
  self.f = theano.function([input],output)
File "<anonymized>/python_modules/Theano/theano/compile/function.py", line 105, in function
  allow_input_downcast=allow_input_downcast)
File "<anonymized>/python_modules/Theano/theano/compile/pfunc.py", line 270, in pfunc
  accept_inplace=accept_inplace, name=name)
File "<anonymized>/python_modules/Theano/theano/compile/function_module.py", line 1105, in orig_function
  fn = Maker(inputs, outputs, mode, accept_inplace = accept_inplace).create(defaults)
File "/u/goodfeli/python_modules/Theano/theano/compile/function_module.py", line 982, in create
  _fn, _i, _o = self.linker.make_thunk(input_storage = input_storage_lists)
File "<anonymized>/python_modules/Theano/theano/gof/link.py", line 321, in make_thunk
  output_storage = output_storage)[:3]
File "<anonymized>/python_modules/Theano/theano/gof/cc.py", line 1178, in make_all
  output_storage = node_output_storage)
File "<anonymized>/python_modules/Theano/theano/gof/cc.py", line 774, in make_thunk
  cthunk, in_storage, out_storage, error_storage = self.__compile__(input_storage, output_storage)
File "<anonymized>/python_modules/Theano/theano/gof/cc.py", line 723, in __compile__
  output_storage)
File "<anonymized>/python_modules/Theano/theano/gof/cc.py", line 1037, in cthunk_factory
  module = get_module_cache().module_from_key(key=key, fn=self.compile_cmodule)
File "<anonymized>/python_modules/Theano/theano/gof/cc.py", line 59, in get_module_cache
  return cmodule.get_module_cache(config.compiledir)
File "<anonymized>/python_modules/Theano/theano/gof/cmodule.py", line 576, in get_module_cache
  _module_cache = ModuleCache(dirname, force_fresh=force_fresh)
File "<anonymized>/python_modules/Theano/theano/gof/cmodule.py", line 268, in __init__
  self.refresh()
File "<anonymized>/python_modules/Theano/theano/gof/cmodule.py", line 326, in refresh
  key = cPickle.load(open(key_pkl, 'rb'))
File "<anonymized>/ops/TiledConvV3D.py", line 504, in <module>
  import TiledConvG3D
File "<anonymized>/ops/TiledConvG3D.py", line 565, in <module>
  import TiledConvC3D
File "<anonymized>/ops/TiledConvC3D.py", line 22, in <module>
  pdb.traceback.print_stack()

此外，我还检查了__builtin__.__import__的id。在我的主脚本的最开始，我导入__builtin__并打印id(__builtin__.__import__)，然后再进行任何其他导入。我还从多次导入的模块内部打印id(__builtin__.import__)，并且id的值不变。

除了调用重载和覆盖__builtin__.__import__之外，是否还有其他机制可以解释我的模块被多次加载的原因？

Answer 1

如果在路径中找到两次Python模块，则可以导入该模块两次。例如，假设您的项目布局如下：

• spam.py
• eggs.py

• 源/
  • 包1

​

​

假设您的PYTHONPATH (sys.path)包含src和src/package1：

PYTHONPATH=/path/to/src:/path/to/src/package1

如果是这样的话，您可以像这样导入同一模块两次：

from package1 import spam
import spam

Python会认为它们是不同的模块。这是怎么回事？

此外，根据下面的讨论(对于搜索此问题的用户)，可以两次导入模块的另一种方法是，如果在第一次导入过程中出现异常。例如，如果垃圾邮件导入鸡蛋，但导入鸡蛋导致模块内部出现异常，则可以再次导入。