Python imports and PYTHONPATH.

Python imports

  • “import foo.bar.x” style:
    • Full name of “foo.bar.x” must be used to refer to what’s imported;
    • Each item except for the last one (“x”) must be a package;
    • The “x” can be a module or a package;
    • The “x” can’t be any of class, function, or variable defined in 2nd last item.
In [1]: import selenium.webdriver.firefox

In [2]: selenium.webdriver.firefox
Out[2]: <module 'selenium.webdriver.firefox' from '/Library/Python/2.7/site-packages/selenium/webdriver/firefox/__init__.pyc'>

In [3]: firefox
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-3-38144139eb6a> in <module>()
----> 1 firefox

NameError: name 'firefox' is not defined
  • “from foo.bar import x” style
    • “x” is now only directly available for use;
    • “x” can be either a module or a subpackage;
    • “x” can also be function, class, variable defined in the module.
In [1]: from selenium.webdriver import firefox

In [2]: firefox
Out[2]: <module 'selenium.webdriver.firefox' from '/Library/Python/2.7/site-packages/selenium/webdriver/firefox/__init__.pyc'>

In [3]: selenium.webdriver.firefox
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-3-f5e8eabbe74b> in <module>()
----> 1 selenium.webdriver.firefox

NameError: name 'selenium' is not defined
  • Package-relative import with . and .. using “from foo.bar import x” style:
# Assuming code in A.B.C module is doing the following imports:
from . import D     # imports A.B.D
from .. import E    # imports A.E
from ..F import G   # imports A.F.G

Python PYTHONPATH

  1. Description:
    • Environment variable like linux PATH;
    • Defines search path for python interpreter to look for modules to import;
    • NOTE that the following are always available in search path:
      • Python installation site-packages directory;
      • Current directory;
      • Current module’s directory (relative import to be possible).
  2. Set PYTHONPATH:
export PYTHONPATH=$PYTHONPATH:/home/user/testdir

References

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