PYTHONPATH Tricks

What is PYTHONPATH

PYTHONPATH is an environment variable that defines where python looks for importable modules

In bash, running echo $PYTHONPATH will show you the current path

Python then adds the directories in that path to sys.path at runtime

Handling of dependencies with multiple versions

If there are multiple versions of a package in sys.path, python searches sys.path in order and uses the first one found

Experiment

To prove this, setup the following scenario

File: ~/package1/package.py def version(): return “Version 1”

File: ~/package2/package.py def version(): return “Version 2”

bash: PYTHONPATH=~/package1/package.py:~/package2/package.py python Inside the python interpreter: import package; package.version() Output: Version 1

This makes sense, running import sys; sys.path shows us that package1 is listed first hence it was loaded

Overriding default resolution behaviors

### Changing default behavior, using path above, package.version() will output Version 1
import os
import sys
MODULE = 'package'

def printPackageVersion():
    import package
    print(f"Currently loaded package version {package.version()}")

if 'package' in sys.modules.keys():
    # If already imported from somewhere else, remove
    printPackageVersion()
    del sys.modules[MODULE]

# Changing to Version 2 of our package
# sys.path was ["", "package1" "package2"] but will be ["", "package2", "package1"] now
sys.path.insert(1, os.path.abspath("~/package2"))
printPackageVersion() # outputs Version2

# What if we wanted to use both versions of the package and use them both in the same file
import importlib.util
spec = importlib.util.spec_from_file_location('package', '~/package1/package.py')
mod = importlib.util.module_from_spec(spec)
spec.loader.exec_module(mod)

mod.version() # Outputs Version 1
package.version() # Outputs Version 2

# What about overriding the already loaded version of the module with our new version
# Now our module versions are out of sync, to hack in Version 1 again globally
del sys.modules[MODULE]
sys.modules[MODULE] = mod

mod.version() # Outputs Version 1
package.version() # Outputs Version 1