How to modify the search path when importing a package in Python

Path search mechanism when importing modules

1.1 Package import search mechanism

Python’s import operation is essentially similar to what we do every day Search the computer for a file with a specified file name. However, Python generally does not search the entire local disk like we do. Instead, it has its own control mechanism that can control the scope and order of the search. This can ensure efficiency and at the same time A certain degree of isolation can also be achieved. For example, python creates a virtual running environment. Based on this principle, different virtual running environments, python installation paths, and import search paths are all isolated separately. Commonly used virtual environment maintenance and creation tools, The core is also achieved by modifying python's default installation path, version, package search path and other information.

Generally, when importing, the package search order and scope are as follows:

1. First search the current folder of the running command, or the current working folder of the system (you can use the command Modify cwd)

2. Then search for the file path in the environment variable PYTHONPATH

3. Finally search for the sitepackages folder in the python installation path (usually through pip All installed packages will be placed in this folder by default. Of course, you can also simply put your own packages in this folder, so there is no need to change the search path)

1.2 Modify the search path

The core idea is to modify the values ​​of the three search nodes mentioned in 1.1 to change the search scope when importing the package

1.2.1 By modifying sys.path Variable

sys.path variable manages the paths of all folders in the Python package search path (i.e. the current folder, the path in the PYTHONPATH environment variable, and the default installation path of python). This variable is a list and can be passed Use methods such as sys.path.append or sys.path.insert to add the path you want to be searched when importing to the list variable.

However, this method will only take effect during runtime and will become invalid once exited.

1.2.2 By modifying the PYTHONPATH environment variable

The PYTHONPATH environment variable is a variable that manages the search path when importing python packages, so it can be modified by adding the path to the environment variable. Search path, and because this method modifies environment variables, it is also permanent, and its impact is global.

For Mac computers, the method to modify this environment variable is as follows:

1. Open the ~/.bashrc file, if it does not exist, create it through the touch command

2. Write export PYTHONPATH=your own dir :$PYTHONPATH in the file. For example, if you want to add the ~/test path to the environment variable, export PYTHONPATH=~/test :$PYTHONPATH

3. Execute source ~/.bashrc on the command line (mainly to let the system reload and run the file to make this modification take effect)

1.2. 3 By adding **.pth file

The above two methods are either temporary modifications, or global modifications have too great impact. The third method can be said to have the characteristics of the first two methods.

You can add a **.pth file in the site-packages directory folder under the specified Python installation directory, and add the path you want to be searched line by line in the file.

Attachment: Method to temporarily add python search package path

Method 1: (Enter python first)

import sys
sys.path.append(‘路径')

(This method is only valid for the current python)

Method 2:

export PYTHONPATH=路径

(This method is valid for all python)

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