Module File Selection

Keep in mind that filename suffixes (e.g., .py) are intentionally omitted from import statements. Python chooses the first file it can find on the search path that matches the imported name. For example, an import statement of the form import b might load:

 

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  • A source code file named b.py
  • A byte code file named b.pyc
  • A directory named b, for package imports (described in Chapter 23)
  • A compiled extension module, usually coded in C or C++ and dynamically linked when imported (e.g., b.so on Linux, or b.dll or b.pyd on Cygwin and Windows)
  • A compiled built-in module coded in C and statically linked into Python
  • A ZIP file component that is automatically extracted when imported
  • An in-memory image, for frozen executables
  • A Java class, in the Jython version of Python
  • A .NET component, in the IronPython version of Python

C extensions, Jython, and package imports all extend imports beyond simple files. To importers, though, differences in the loaded file type are completely transparent, both when importing and when fetching module attributes. Saying import b gets whatever module b is, according to your module search path, and b.attr fetches an item in the module, be it a Python variable or a linked-in C function. Some standard modules we will use in this book are actually coded in C, not Python; because of this transparency, their clients don’t have to care.

If you have both a b.py and a b.so in different directories, Python will always load the one found in the first (leftmost) directory of your module search path during the left-to-right search of sys.path. But what happens if it finds both a b.py and a b.so in the same directory? In this case, Python follows a standard picking order, though this order is not guaranteed to stay the same over time. In general, you should not depend on which type of file Python will choose within a given directory—make your module names distinct, or configure your module search path to make your module selection preferences more obvious.