Python set sys path

How to temporarily modify sys.path in python?

In Python, the sys.path variable is used to specify the search path for modules. The default sys.path includes the current directory, the standard library, and the site-packages directory. However, sometimes it may be necessary to temporarily modify the sys.path in order to include additional directories or to change the order in which directories are searched.

Method 1: Using sys.path.insert()

To temporarily modify sys.path in Python, you can use the sys.path.insert() method. This method inserts a new path at the beginning of the list of paths in sys.path . Here’s how to use it:

import sys sys.path.insert(0, '/path/to/new/folder') import my_module

In the above example, we first import the sys module. We then use the sys.path.insert() method to add a new path to sys.path . The first argument to this method is the index at which to insert the new path (in this case, we want it to be the first path in the list, so we use 0 ). The second argument is the path to be inserted.

Once we’ve added the new path, we can import modules from it just like we would with any other path. In the example above, we import a module called my_module from the new path.

import sys original_path = sys.path sys.path.insert(0, '/path/to/new/folder') import my_module sys.path = original_path

In this example, we first save the original sys.path to a variable called original_path . We then add a new path to sys.path using sys.path.insert() . We import a module from the new path, just like before.

Finally, we restore the original sys.path by assigning original_path back to sys.path .

This method can be useful when you need to temporarily modify sys.path for a specific task, without affecting the rest of your code.

Method 2: Using sys.path.append()

To temporarily modify sys.path in Python, you can use the sys.path.append() method. This method appends a new directory to the list of directories in sys.path .

import sys sys.path.append('/path/to/new/directory') import mymodule

In this example, we import the sys module and use the sys.path.append() method to add a new directory to sys.path . We then import a module from the new directory using the import statement.

You can also use the os.path.join() method to construct the path to the new directory:

import sys import os new_dir = os.path.join('/path', 'to', 'new', 'directory') sys.path.append(new_dir) import mymodule

In this example, we use the os.path.join() method to construct the path to the new directory, and then use sys.path.append() to add the new directory to sys.path . We then import a module from the new directory using the import statement.

It’s important to note that sys.path is only modified temporarily for the current Python session. If you exit the Python session and start a new one, sys.path will be reset to its original value.

That’s it! Using sys.path.append() is a simple and effective way to temporarily modify sys.path in Python.

Method 3: Using sys.path.extend()

To temporarily modify sys.path in Python, you can use the sys.path.extend() method. This method allows you to add new directories to the Python module search path at runtime.

import sys print(sys.path) sys.path.extend(['/path/to/new/directory']) print(sys.path)

In this example, we import the sys module and print the current value of sys.path . We then use sys.path.extend() to add a new directory to the module search path. Finally, we print the updated value of sys.path .

You can also use os.path.join() to construct the new directory path dynamically:

import sys import os print(sys.path) new_dir = os.path.join(os.getcwd(), 'new_directory') sys.path.extend([new_dir]) print(sys.path)

In this example, we use the os module to construct the path to a new directory called «new_directory» located in the current working directory. We then use sys.path.extend() to add the new directory to the module search path.

Note that modifications made to sys.path using sys.path.extend() are temporary and will only exist for the duration of the Python session. If you want to make permanent changes to the module search path, you should modify the PYTHONPATH environment variable or update the site.py file.

Method 4: Using PYTHONPATH environment variable

One way to temporarily modify sys.path in Python is by using the PYTHONPATH environment variable. This variable allows you to specify a list of directories that Python will search for modules when it starts up.

Here’s an example of how to modify sys.path using PYTHONPATH :

import os import sys os.environ['PYTHONPATH'] = '/path/to/new/directory:' + os.environ.get('PYTHONPATH', '') import mymodule

1. First, we import the os and sys modules.
2. Next, we set the PYTHONPATH environment variable to include the path to the new directory we want to add to sys.path . We do this by appending the path to the existing value of PYTHONPATH , if it exists.
3. Finally, we can import modules from the new directory as if they were in the original sys.path .

Note that this method only modifies sys.path temporarily for the current Python process. If you want to make the change permanent, you’ll need to modify your system’s environment variables.

Step-by-Step Instructions

1. Determine the path to the directory you want to add to sys.path .
2. Open a Python script or shell.
3. Import the os and sys modules.
4. Set the PYTHONPATH environment variable using os.environ[‘PYTHONPATH’] = ‘/path/to/new/directory:’ + os.environ.get(‘PYTHONPATH’, ») .
5. Import modules from the new directory as if they were in the original sys.path .

Method 5: Using .pth file

To temporarily modify the sys.path in Python, one way is to use a .pth file. A .pth file is a plain text file that contains a list of directories to be added to the sys.path when Python starts up. Here are the steps to use a .pth file:

Create a plain text file with a .pth extension, such as mylibs.pth . Inside the file, add one directory per line, like this:

/home/user/mylibs /home/user/otherlibs

The site-packages directory is where Python looks for third-party modules. You can find the location of the site-packages directory by running the following command in Python:

import site print(site.getsitepackages())

The output will be a list of directories, and the site-packages directory is one of them. Place your .pth file in the site-packages directory.

To test the modification, you can run the following code:

The output should include the directories you added in the .pth file.

Note that the modification is temporary and only affects the current Python session. If you want to make the modification permanent, you can add the directories to the PYTHONPATH environment variable or modify the sys.path in your Python script.

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The initialization of the sys.path module search path¶

A module search path is initialized when Python starts. This module search path may be accessed at sys.path .

The first entry in the module search path is the directory that contains the input script, if there is one. Otherwise, the first entry is the current directory, which is the case when executing the interactive shell, a -c command, or -m module.

The PYTHONPATH environment variable is often used to add directories to the search path. If this environment variable is found then the contents are added to the module search path.

PYTHONPATH will affect all installed Python versions/environments. Be wary of setting this in your shell profile or global environment variables. The site module offers more nuanced techniques as mentioned below.

The next items added are the directories containing standard Python modules as well as any extension module s that these modules depend on. Extension modules are .pyd files on Windows and .so files on other platforms. The directory with the platform-independent Python modules is called prefix . The directory with the extension modules is called exec_prefix .

The PYTHONHOME environment variable may be used to set the prefix and exec_prefix locations. Otherwise these directories are found by using the Python executable as a starting point and then looking for various ‘landmark’ files and directories. Note that any symbolic links are followed so the real Python executable location is used as the search starting point. The Python executable location is called home .

Once home is determined, the prefix directory is found by first looking for python majorversion minorversion .zip ( python311.zip ). On Windows the zip archive is searched for in home and on Unix the archive is expected to be in lib . Note that the expected zip archive location is added to the module search path even if the archive does not exist. If no archive was found, Python on Windows will continue the search for prefix by looking for Lib\os.py . Python on Unix will look for lib/python majorversion . minorversion /os.py ( lib/python3.11/os.py ). On Windows prefix and exec_prefix are the same, however on other platforms lib/python majorversion . minorversion /lib-dynload ( lib/python3.11/lib-dynload ) is searched for and used as an anchor for exec_prefix . On some platforms lib may be lib64 or another value, see sys.platlibdir and PYTHONPLATLIBDIR .

Once found, prefix and exec_prefix are available at sys.prefix and sys.exec_prefix respectively.

Finally, the site module is processed and site-packages directories are added to the module search path. A common way to customize the search path is to create sitecustomize or usercustomize modules as described in the site module documentation.

Certain command line options may further affect path calculations. See -E , -I , -s and -S for further details.

Virtual environments¶

If Python is run in a virtual environment (as described at Virtual Environments and Packages ) then prefix and exec_prefix are specific to the virtual environment.

If a pyvenv.cfg file is found alongside the main executable, or in the directory one level above the executable, the following variations apply:

• If home is an absolute path and PYTHONHOME is not set, this path is used instead of the path to the main executable when deducing prefix and exec_prefix .

_pth files¶

To completely override sys.path create a ._pth file with the same name as the shared library or executable ( python._pth or python311._pth ). The shared library path is always known on Windows, however it may not be available on other platforms. In the ._pth file specify one line for each path to add to sys.path . The file based on the shared library name overrides the one based on the executable, which allows paths to be restricted for any program loading the runtime if desired.

When the file exists, all registry and environment variables are ignored, isolated mode is enabled, and site is not imported unless one line in the file specifies import site . Blank paths and lines starting with # are ignored. Each path may be absolute or relative to the location of the file. Import statements other than to site are not permitted, and arbitrary code cannot be specified.

Note that .pth files (without leading underscore) will be processed normally by the site module when import site has been specified.

Embedded Python¶

If Python is embedded within another application Py_InitializeFromConfig() and the PyConfig structure can be used to initialize Python. The path specific details are described at Python Path Configuration . Alternatively the older Py_SetPath() can be used to bypass the initialization of the module search path.

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