Modules
If you quit from the Python interpreter and enter it again, the definitions you have made (functions and variables) are lost. Therefore, if you want to write a somewhat longer program, you are better off using a text editor to prepare the input for the interpreter and running it with that file as input instead. This is known as creating a script. As your program gets longer, you may want to split it into several files for easier maintenance. You may also want to use a handy function that you’ve written in several programs without copying its definition into each program.
To support this, Python has a way to put definitions in a file and use them in a script or in an interactive instance of the interpreter. Such a file is called a module; definitions from a module can be imported into other modules or into the main module (the collection of variables that you have access to in a script executed at the top level and in calculator mode).
A module is a file containing Python definitions and statements. The file name is the module name with the suffix .py appended. Within a module, the module’s name (as a string) is available as the value of the global variable __name__ . For instance, use your favorite text editor to create a file called fibo.py in the current directory with the following contents:
# Fibonacci numbers module def fib(n): # write Fibonacci series up to n a, b = 0, 1 while a < n: print(a, end=' ') a, b = b, a+b print() def fib2(n): # return Fibonacci series up to n result = [] a, b = 0, 1 while a < n: result.append(a) a, b = b, a+b return result
Now enter the Python interpreter and import this module with the following command:
This does not add the names of the functions defined in fibo directly to the current namespace (see Python Scopes and Namespaces for more details); it only adds the module name fibo there. Using the module name you can access the functions:
>>> fibo.fib(1000) 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 >>> fibo.fib2(100) [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89] >>> fibo.__name__ 'fibo'
If you intend to use a function often you can assign it to a local name:
>>> fib = fibo.fib >>> fib(500) 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
6.1. More on Modules
A module can contain executable statements as well as function definitions. These statements are intended to initialize the module. They are executed only the first time the module name is encountered in an import statement. 1 (They are also run if the file is executed as a script.)
Each module has its own private namespace, which is used as the global namespace by all functions defined in the module. Thus, the author of a module can use global variables in the module without worrying about accidental clashes with a user’s global variables. On the other hand, if you know what you are doing you can touch a module’s global variables with the same notation used to refer to its functions, modname.itemname .
Modules can import other modules. It is customary but not required to place all import statements at the beginning of a module (or script, for that matter). The imported module names, if placed at the top level of a module (outside any functions or classes), are added to the module’s global namespace.
There is a variant of the import statement that imports names from a module directly into the importing module’s namespace. For example:
>>> from fibo import fib, fib2 >>> fib(500) 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
This does not introduce the module name from which the imports are taken in the local namespace (so in the example, fibo is not defined).
There is even a variant to import all names that a module defines:
>>> from fibo import * >>> fib(500) 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
This imports all names except those beginning with an underscore ( _ ). In most cases Python programmers do not use this facility since it introduces an unknown set of names into the interpreter, possibly hiding some things you have already defined.
Note that in general the practice of importing * from a module or package is frowned upon, since it often causes poorly readable code. However, it is okay to use it to save typing in interactive sessions.
If the module name is followed by as , then the name following as is bound directly to the imported module.
>>> import fibo as fib >>> fib.fib(500) 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
This is effectively importing the module in the same way that import fibo will do, with the only difference of it being available as fib .
It can also be used when utilising from with similar effects:
>>> from fibo import fib as fibonacci >>> fibonacci(500) 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
For efficiency reasons, each module is only imported once per interpreter session. Therefore, if you change your modules, you must restart the interpreter – or, if it’s just one module you want to test interactively, use importlib.reload() , e.g. import importlib; importlib.reload(modulename) .
6.1.1. Executing modules as scripts
When you run a Python module with
the code in the module will be executed, just as if you imported it, but with the __name__ set to «__main__» . That means that by adding this code at the end of your module:
if __name__ == "__main__": import sys fib(int(sys.argv[1]))
you can make the file usable as a script as well as an importable module, because the code that parses the command line only runs if the module is executed as the “main” file:
$ python fibo.py 50 0 1 1 2 3 5 8 13 21 34
If the module is imported, the code is not run:
This is often used either to provide a convenient user interface to a module, or for testing purposes (running the module as a script executes a test suite).
6.1.2. The Module Search Path
When a module named spam is imported, the interpreter first searches for a built-in module with that name. These module names are listed in sys.builtin_module_names . If not found, it then searches for a file named spam.py in a list of directories given by the variable sys.path . sys.path is initialized from these locations:
- The directory containing the input script (or the current directory when no file is specified).
- PYTHONPATH (a list of directory names, with the same syntax as the shell variable PATH ).
- The installation-dependent default (by convention including a site-packages directory, handled by the site module).
On file systems which support symlinks, the directory containing the input script is calculated after the symlink is followed. In other words the directory containing the symlink is not added to the module search path.
After initialization, Python programs can modify sys.path . The directory containing the script being run is placed at the beginning of the search path, ahead of the standard library path. This means that scripts in that directory will be loaded instead of modules of the same name in the library directory. This is an error unless the replacement is intended. See section Standard Modules for more information.
6.1.3. “Compiled” Python files
To speed up loading modules, Python caches the compiled version of each module in the __pycache__ directory under the name module.version.pyc , where the version encodes the format of the compiled file; it generally contains the Python version number. For example, in CPython release 3.3 the compiled version of spam.py would be cached as __pycache__/spam.cpython-33.pyc . This naming convention allows compiled modules from different releases and different versions of Python to coexist.
Python checks the modification date of the source against the compiled version to see if it’s out of date and needs to be recompiled. This is a completely automatic process. Also, the compiled modules are platform-independent, so the same library can be shared among systems with different architectures.
Python does not check the cache in two circumstances. First, it always recompiles and does not store the result for the module that’s loaded directly from the command line. Second, it does not check the cache if there is no source module. To support a non-source (compiled only) distribution, the compiled module must be in the source directory, and there must not be a source module.
- You can use the -O or -OO switches on the Python command to reduce the size of a compiled module. The -O switch removes assert statements, the -OO switch removes both assert statements and __doc__ strings. Since some programs may rely on having these available, you should only use this option if you know what you’re doing. “Optimized” modules have an opt- tag and are usually smaller. Future releases may change the effects of optimization.
- A program doesn’t run any faster when it is read from a .pyc file than when it is read from a .py file; the only thing that’s faster about .pyc files is the speed with which they are loaded.
- The module compileall can create .pyc files for all modules in a directory.
- There is more detail on this process, including a flow chart of the decisions, in PEP 3147.
6.2. Standard Modules
Python comes with a library of standard modules, described in a separate document, the Python Library Reference (“Library Reference” hereafter). Some modules are built into the interpreter; these provide access to operations that are not part of the core of the language but are nevertheless built in, either for efficiency or to provide access to operating system primitives such as system calls. The set of such modules is a configuration option which also depends on the underlying platform. For example, the winreg module is only provided on Windows systems. One particular module deserves some attention: sys , which is built into every Python interpreter. The variables sys.ps1 and sys.ps2 define the strings used as primary and secondary prompts:
>>> import sys >>> sys.ps1 '>>> ' >>> sys.ps2 '. ' >>> sys.ps1 = 'C> ' C> print('Yuck!') Yuck! C>
These two variables are only defined if the interpreter is in interactive mode.
The variable sys.path is a list of strings that determines the interpreter’s search path for modules. It is initialized to a default path taken from the environment variable PYTHONPATH , or from a built-in default if PYTHONPATH is not set. You can modify it using standard list operations:
>>> import sys >>> sys.path.append('/ufs/guido/lib/python')
Python 3.11
The Python interpreter is usually installed as /usr/local/bin/python3.11 those machines where it available; putting your Unix shell’s search path makes
In the following examples, input and output are distinguished by presence or absence of prompts to repeat you must type everything after when appears;
The os module provides dozens of functions for interacting with operating system: Be sure to use the import os style instead of from The built-in dir()