How to temporarily modify sys.path in Python?

Question

In one of my testing scripts in Python I use this pattern several times:

sys.path.insert(0, "somedir")
mod =  __import__(mymod)
sys.path.pop(0)

Is there a more concise way to temporarily modify the search path?

Asked By: planetp

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Source

Answer 1

Appending a value to sys.path only modifies it temporarily, i.e for that session only.

Permanent modifications are done by changing PYTHONPATH and the default installation directory.

So, if by temporary you meant for current session only then your approach is okay, but you can remove the pop part if somedir is not hiding any important modules that is expected to be found in in PYTHONPATH ,current directory or default installation directory.

http://docs.python.org/2/tutorial/modules.html#the-module-search-path

Answered By: Ashwini Chaudhary

Answer 2

You could use a simple context manager:

import sys

class add_path():
    def __init__(self, path):
        self.path = path

    def __enter__(self):
        sys.path.insert(0, self.path)

    def __exit__(self, exc_type, exc_value, traceback):
        try:
            sys.path.remove(self.path)
        except ValueError:
            pass

Then to import a module you can do:

with add_path('/path/to/dir'):
    mod = __import__('mymodule')

On exit from the body of the with statement sys.path will be restored to the original state. If you only use the module within that block you might also want to delete its reference from sys.modules:

del sys.modules['mymodule']

Answered By: Eugene Yarmash

Answer 3

Here is an alternative implementation of the contextmanager implementation from Eugene Yarmash (use contextlib and pathlib.Path-compatible):

import os
import sys
import contextlib
from typing import Iterator, Union

@contextlib.contextmanager
def add_sys_path(path: Union[str, os.PathLike]) -> Iterator[None]:
    """Temporarily add the given path to `sys.path`."""
    path = os.fspath(path)
    try:
        sys.path.insert(0, path)
        yield
    finally:
        sys.path.remove(path)


with add_sys_path('/path/to/dir'):
    mymodule = importlib.import_module('mymodule')

Answered By: Conchylicultor

Answer 4

If you’re testing uses pytest, they have a great fixture that, among other traits, handles this exact case:

The monkeypatch fixture helps you to safely set/delete an attribute,
dictionary item or environment variable, or to modify sys.path for
importing…
All modifications will be undone after the requesting test function or
fixture has finished. The raising parameter determines if a KeyError
or AttributeError will be raised if the target of the set/deletion
operation does not exist

In describing syspath_prepend:

Use monkeypatch.syspath_prepend to modify sys.path which will also
call pkg_resources.fixup_namespace_packages and
importlib.invalidate_caches().

sample use:

def test_myfunc(monkeypatch):
  with monkeypatch.context() as m:
    m.syspath_prepend('my/module/path')
    mod =  __import__(mymod)
  # Out here, the context manager expires and syspath is reset

Answered By: ntjess

How to temporarily modify sys.path in Python?

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