python - 导入错误 : No module named package

Question

我发现在 Python 中导入模块很复杂，所以我正在做实验来解决这个问题。这是我的文件结构:

PythonTest/
  package/
    __init__.py
    test.py

__init__.py的内容:

package = 'Variable package in __init__.py'
from package import test

test.py内容:

from package import package
print package

当我离开 package(在 PythonTest 中)并执行 python package/test.py 时，我得到:

Traceback (most recent call last):
  File "package/test.py", line 1, in <module>
    from package import package
ImportError: No module named package

预期的输出是Variable package in __init__.py。我做错了什么？

---

但是，我可以在交互模式下得到预期的输出:

sunqingyaos-MacBook-Air:PythonTest sunqingyao$ python
Python 2.7.10 (default, Oct 23 2015, 19:19:21) 
[GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.0.59.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import package
Package in __init__.py

Answer 1

首先让我们看看Python是如何搜索包和模块的。 sys.path

A list of strings that specifies the search path for modules. Initialized from the environment variable PYTHONPATH, plus an installation-dependent default.

那是搜索路径。因此，如果您的模块/包位于 sys.path 之一，python 解释器能够找到并导入它。文档说得更多:

As initialized upon program startup, the first item of this list, path[0], is the directory containing the script that was used to invoke the Python interpreter. If the script directory is not available (e.g. if the interpreter is invoked interactively or if the script is read from standard input), path[0] is the empty string, which directs Python to search modules in the current directory first.

我修改了test.py作为例子。

import sys; import pprint
pprint.pprint(sys.path)

from package import package
print package 

有两种情况:

$ python package/test.py
['/Users/laike9m/Dev/Python/TestPython/package',
 '/usr/local/lib/python2.7/site-packages/doc2dash-2.1.0.dev0-py2.7.egg',
 '/usr/local/lib/python2.7/site-packages/zope.interface-4.1.3-py2.7-macosx-10.10-x86_64.egg',
 '/usr/local/lib/python2.7/site-packages/six-1.10.0-py2.7.egg',
 '/usr/local/lib/python2.7/site-packages/colorama-0.3.3-py2.7.egg',

如你所见，path[0]是/Users/laike9m/Dev/Python/TestPython/package，这是包含脚本test的目录.py 用于调用 Python 解释器。

$ python                                         
Python 2.7.12 (default, Jun 29 2016, 14:05:02)
[GCC 4.2.1 Compatible Apple LLVM 7.3.0 (clang-703.0.31)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import package
['',
 '/usr/local/lib/python2.7/site-packages/doc2dash-2.1.0.dev0-py2.7.egg',
 '/usr/local/lib/python2.7/site-packages/zope.interface-4.1.3-py2.7-macosx-10.10-x86_64.egg',
 '/usr/local/lib/python2.7/site-packages/six-1.10.0-py2.7.egg',
 '/usr/local/lib/python2.7/site-packages/colorama-0.3.3-py2.7.egg',
...

现在是第二种情况，当交互式调用时，“path[0] 是一个空字符串，它指示 Python 首先在当前目录中搜索模块。”当前目录是什么？ /Users/laike9m/Dev/Python/TestPython/。(看这是我机器上的路径，在你的情况下它等同于 PythonTest 的路径)

现在你知道答案了:

1. 为什么 python package/test.py 出现 ImportError: No module named package?

   因为解释器没有“看到”包。要让解释器知道包 package，PythonTest 必须在 sys.path 中，但事实并非如此。
   
   

2. 为什么这在交互模式下有效？

   因为现在 PythonTest 位于 sys.path 中，所以解释器能够找到包 package。