In this chapter we are going to learn about Python modules.
Up until now, all the code we wrote in the Python interpreter was lost when we exited the interpreter. But when people write large programs they tend to break their code into multiple different files for ease of use, debugging and readability. In Python we use modules to achieve such goals. Modules are nothing but files with Python definitions and statements. The module name, to import, has the same name of the Python file without the .py extension.
You can find the name of the module by accessing the __name__ variable. It is a global variable.
Now we are going to see how modules work. Create a file called bars.py. Content of the file is given bellow.
""" Bars Module ============ This is an example module with provide different ways to print bars. """ def starbar(num): """Prints a bar with * :arg num: Length of the bar """ print('*' * num) def hashbar(num): """Prints a bar with # :arg num: Length of the bar """ print('#' * num) def simplebar(num): """Prints a bar with - :arg num: Length of the bar """ print('-' * num)
Now we are going to start the Python interpreter and import our module.
>>> import bars >>>
This will import the module bars. We have to use the module name to access functions inside the module.
>>> bars.hashbar(10) ########## >>> bars.simplebar(10) ---------- >>> bars.starbar(10) **********
There are different ways to import modules. We already saw one way to do this. You can even import selected functions from modules. To do so:
>>> from bars import simplebar, starbar >>> simplebar(20) --------------------
Never do from module import * Read this link for more information.
We can have many submodules inside a module. A directory with a __init__.py can also be used as a module and all .py files inside it become submodules.
$ tree mymodule mymodule |-- bars.py |-- __init__.py `-- utils.py
In this example mymodule is the module name and bars and utils are two submodules in it. You can create an empty __init__.py using touch command.
$ touch mymodule/__init__.py
__all__ in __init__.py¶
If __init__.py file contains a list called __all__, then only the names listed there will be public. So if the mymodule’s __init__.py file contains the following
from mymodule.bars import simplebar __all__ = [simplebar, ]
Then from mymodule only simplebar will be available.
from mymodule import * will only work for module level objects, trying to use it to import functions or classes will cause syntax error.
Now your Python installation comes with different modules installed, you can use them as required and install new modules for any other special purposes. In the following few examples we are going to see many examples on the same.
>>> help() Welcome to Python 3.5's help utility! If this is your first time using Python, you should definitely check out the tutorial on the Internet at http://docs.python.org/3.5/tutorial/. Enter the name of any module, keyword, or topic to get help on writing Python programs and using Python modules. To quit this help utility and return to the interpreter, just type "quit". To get a list of available modules, keywords, symbols, or topics, type "modules", "keywords", "symbols", or "topics". Each module also comes with a one-line summary of what it does; to list the modules whose name or summary contain a given string such as "spam", type "modules spam". help> modules
The above example shows how to get the list of all installed modules in your system. I am not pasting them here as it is a big list in my system :)
You can also use help() function in the interpeter to find documentation about any module/classes. Say you want to know all available methods in strings, you can use the following method
os module provides operating system dependent functionality. You can import it using the following import statement.
>>> import os
getuid() function returns the current process’s effective user’s id.
>>> os.getuid() 500
getpid() returns the current process’s id. getppid() returns the parent process’s id.
>>> os.getpid() 16150 >>> os.getppid() 14847
uname() returns different information identifying the operating system, in Linux it returns details you can get from the uname command. The returned object is a tuple, (sysname, nodename, release, version, machine)
>>> os.uname() ('Linux', 'd80', '188.8.131.52-56.fc13.i686.PAE', '#1 SMP Wed Sep 15 03:27:15 UTC 2010', 'i686')
getcwd()*returns the current working directory. *chdir(path) changes the current working directory to path. In the example we first see the current directory which is my home directory and change the current directory to /tmp and then again checking the current directory.
>>> os.getcwd() '/home/kushal' >>> os.chdir('/tmp') >>> os.getcwd() '/tmp'
So let us use another function provided by the os module and create our own function to list all files and directories in any given directory.
def view_dir(path='.'): """ This function prints all files and directories in the given directory. :args path: Path to the directory, default is current directory """ names = os.listdir(path) names.sort() for name in names: print(name, end =' ')
Using the view_dir example.
>>> view_dir('/') .readahead bin boot dev etc home junk lib lib64 lost+found media mnt opt proc root run sbin srv sys tmp usr var
There are many other very useful functions available in the OS module, you can read about them here
requests is a Python module which changed the way people used to write code for many many projects. It helps you to do HTTP GET or POST calls in a very simple but elegant way. This is a third party module, that means you have to install it from your OS distribution packages, it does not come default.
# yum install python3-requests
The above command will install Python3 version of the requests module in your system.
Getting a simple web pages¶
You can use the get method to fetch any website.
>>> import requests >>> req = requests.get('http://google.com') >>> req.status_code 200
The text attribute holds the HTML returned by the server.
Using this knowledge, let us write a command which can download a given file (URL) from Internet.
#!/usr/bin/env python3 import os import os.path import requests def download(url): '''Download the given url and saves it to the current directory. :arg url: URL of the file to be downloaded. ''' req = requests.get(url) # First let us check non existing files. if req.status_code == 404: print('No such file found at %s' % url) return filename = url.split('/')[-1] with open(filename, 'wb') as fobj: fobj.write(req.content) print("Download over.") if __name__ == '__main__': url = input('Enter a URL:') download(url)
Here we used something new, when the module name is __main__, then only ask for a user input and then download the given URL. This also prevents the same when some other Python code imports this file as a Python module.
To learn more about requests module, go to their wonderful documentation.
You can actually modify the above program to become more user friendly. For example, you can check if that given
filename already exists in the current directory or not. Use
os.path module for the name.
Command line arguments¶
Do you remember your ls command, you can pass different kind of options as command line arguments. You can do that too .. important:: your application. Read this how-to guide to learn about it.
TAB completion in your Python interpreter¶
First create a file as ~/.pythonrc and include the following in that file
import rlcompleter, readline readline.parse_and_bind('tab: complete') history_file = os.path.expanduser('~/.python_history') readline.read_history_file(history_file) import atexit atexit.register(readline.write_history_file, history_file)
Next, just export PYTHONSTARTUP variable pointing to this file from your ~/.bashrc file.
Now from future whenever you open a bash shell, you will have TAB completion and history of code entered in your Python interpreter.
To use it in the current shell, source the bashrc file.
$ source ~/.bashrc