Friday, March 25, 2016

How-to: When a Missing Python Module Error Was Thrown

When updating Python from 2 to 3, you may want to get familiar with the following topics first:
  1. Can you install multiple Python versions in Linux?
  2. How to do when a missing module error was thrown?
  3. Learn about search path to locate modules in Python
  4. Know the differences between Python 2 and 3[1]
  5. How to resolve missing Python module 
    • ImportError: No module named 'encodings'
  6. Where is a specific Python module located?

Multiple Python Installations


In our system, we have both Python 2 and 3 installed under /usr/bin as such:
  • /usr/bin/python
  • /usr/bin/python3
To choose a specific version to use in your python scripts, you can specify shebang (or hashbang) as follows:
#!/usr/bin/python3

Python2

$ python
Python 2.4.3 (#1, Feb 24 2012, 13:04:26)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-50)] on linux2
Type "help", "copyright", "credits" or "license" for more information.


Python3

$ python3
Python 3.5.1 (default, Mar 24 2016, 20:01:47)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-52)] on linux
Type "help", "copyright", "credits" or "license" for more information.

In the rest of article, we will use Python 3.5.1 for illustration unless stated otherwise.  Read section "Python 2 vs Python 3" to learn the differences between them.

Missing Python Module


Python module is a file (e.g., with suffixes like .py.pyc, .pyo etc.):[13,17]
  • Containing Python definitions and statements 
  • Can be imported in a script or in an interactive instance of the interpreter
    • Imported only once per interpreter session
      • Simply for efficiency reasons
      • If you change your modules, you must restart the interpreter
      • If it’s just one module you want to test interactively, can also use importlib.reload().[14]
Oftentimes, you could run into missing Python Module reported by ImportError module like:

$ python3.5
Fatal Python error: Py_Initialize: Unable to get the locale encoding
ImportError: No module named 'encodings'
In such cases, you may need to fix sys.path to include missing library paths.

sys.path


sys.path variable stores a list of strings that specifies the search path for modules. It is initialized from these locations:
  1. The directory containing the input script (or the current directory when no file is specified)
  2. PYTHONPATH (a list of directory names)
    • With the same syntax as the shell variable PATH
  3. The installation-dependent default

A program is free to modify this list for its own purposes. Only strings and bytes should be added to sys.path; all other data types are ignored during import.  See also Module site — This describes how to use .pth files to extend sys.path.

Python 2 vs 3  


In this section, we will show you how to display sys.path value from the command line without entering interactive mode.  To do that, we use a built-in module print.  However, as noted below, there are syntax differences between Python 2 and Python 3 in the way of invoking it.


Python 2

$ python -c 'import sys; print "\n".join(sys.path)'
/usr/lib64/python24.zip
/usr/lib64/python2.4
/usr/lib64/python2.4/plat-linux2
/usr/lib64/python2.4/lib-tk
/usr/lib64/python2.4/lib-dynload
/usr/lib64/python2.4/site-packages
/usr/lib64/python2.4/site-packages/Numeric
/usr/lib64/python2.4/site-packages/PIL
/usr/lib64/python2.4/site-packages/gtk-2.0
/usr/lib/python2.4/site-packages

Python 3

$ python3 -c 'import sys; print("\n".join(sys.path))'
/usr/lib/python35.zip
/usr/lib/python3.5
/usr/lib/python3.5/plat-linux
/usr/lib/python3.5/lib-dynload
/scratch/perf/.local/lib/python3.5/site-packages
/usr/lib/python3.5/site-packages

Where Is a Python Module Located?


When a module, say,  encodings is imported, the interpreter first searches for a built-in module with that name. If not found, it then searches for a file named encodings.py in a list of directories given by the variable sys.path.

To find out where an imported module is located, you can use its attribute __file__.[11]  For example, module encodings is located under /usr/lib/python3.5 in our system:

$ python3
<snipped>
>>> import encodings
>>> print(encodings.__file__)
/usr/lib/python3.5/encodings/__init__.py

References

  1. What does “SyntaxError: Missing parentheses in call to 'print'” mean in Python?
    • This error message means that you are attempting to use Python 3 to follow an example or run a program that uses the Python 2 print statement:
  2. Install / Update Python 3.5.0 at Linux machine. (Youtube)
  3. Python 3.5.1 
  4. Python Module
  5. upgrade Python to 2.7.2
  6. How can I troubleshoot Python “Could not find platform independent libraries
  7. Py_Initialize: Unable to get the locale encoding in OpenSuse 12.3
  8. Environment Variables (Python)
  9. Python script header
  10. Standard modules (Python)
  11. How do I find the location of Python module sources?
  12. sys module — System-specific parameters and functions
  13. What do the python file extensions, .pyc .pyd .pyo stand for?
  14. How do I unload (reload) a Python module?
  15. Purpose of #!/usr/bin/python3 (important)
  16. shebang (or hashbang)
    • Under Unix-like operating systems, when a script with a shebang is run as a program, the program loader parses the rest of the script's initial line as an interpreter directive; the specified interpreter program is run instead, passing to it as an argument the path that was initially used when attempting to run the script.
  17. Importing Python Modules



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