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planetp
planetp

Reputation: 16113

How to temporarily modify sys.path in Python?

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?

Upvotes: 40

Views: 35881

Answers (4)

Conchylicultor
Conchylicultor

Reputation: 5739

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')

Upvotes: 8

ntjess
ntjess

Reputation: 1480

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

Upvotes: 4

Eugene Yarmash
Eugene Yarmash

Reputation: 150128

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']

Upvotes: 40

Ashwini Chaudhary
Ashwini Chaudhary

Reputation: 251146

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

Upvotes: 21

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