Why does << export PYTHONPATH=$PYTHONPATH >> put the current working directory on sys.path?

Question

Put that line in .bashrc (.profile on Mac), the working directory is on the path. Leave it out, it isn't.

export PYTHONPATH=$PYTHONPATH

I've noticed that shells with the line have a PYTHONPATH= line in their env output, those without don't, so perhaps that matters. Python also adds the current working directory if PYTHONPATH has some valid directory but begins with a ':' character. (Simply setting PYTHONPATH to some valid directory breaks the working directory inclusion.)

Why is this so? What is best practice for ensuring the current working directory is on sys.path?

Answer 1

If you want to ensure it, then explicitly add . to the pythonpath:

export PYTHONPATH=.:$OTHER_DIRS:$PYTHONPATH

If you also want to have the current working directory in your sys.path with a PYTHONPATH set to something else, then you could try to set it up in your sitecustomize.py or usercustomize.py by inserting something like:

sys.path.append(os.getcwd())

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Additional note:

Since the introduction of per user site-packages it's best to put your user packages there (you can get the folder name with site.USER_SITE). pip also makes it easy to install packages thera using the --user option, so I've found that at least for me ther rarely is the necessity to mess around with PYTHONPATH.

Answer 2

As per the docs,(this should clear your doubts)

The PYTHONPATH IS AN ENV. VARIABLE THAT augments the default search path for module files. The format is the same as the shell’s PATH: one or more directory pathnames separated by os.pathsep (e.g. colons on Unix or semicolons on Windows). Non-existent directories are silently ignored.

In addition to normal directories, individual PYTHONPATH entries may refer to zipfiles containing pure Python modules (in either source or compiled form). Extension modules cannot be imported from zipfiles.

The default search path is installation dependent, but generally begins with prefix/lib/pythonversion (see PYTHONHOME above). It is always appended to PYTHONPATH.

An additional directory will be inserted in the search path in front of PYTHONPATH as described above under Interface options. The search path can be manipulated from within a Python program as the variable sys.path.