Python For Programmers Part 3

May 01, 2021

This is a series on Python and how to correctly use Python when coming from a background in another computer language. Because of this, this will not be a slow intro into programming and it will be assumed you have a preferred text editor and are smart enough to get Python running. You can download installers and packages from their official website found here. Basic knowledge of how to use Git and how to operate a computer is also preferred.

This series will assume you are familiar with some form of a computer language and basic programming paradigms. C/C++, JavaScript, and Go will be referred to most as they are my most familiar languages other than Python, but anyone with experience in really anything else should be able to follow along.

Also I know Python calls them methods but I use “methods” and “function” interchangeably.

Classes

There is something I omitted from the last article that you experienced object-oriented programmers and hackers would probably be in anguish without: classes. Classes are very engrained in Python’s nature. Creating a class can be done like so.

# This file is named "dogs.py" class Dog: # this is a class variable and will be shared # with all instances of class 'Dog' genus = 'canine' # this function gets called # When you initialize a new instance # of the class 'Dog' # these variables are on a per instance # basis and will not be shared. def __init__(self, name, age): self.age = age self.name = name self.parts = {} self.furry = True self.hypoallergenic = False # this will be called when the # class needs to be represented # such as when you try to print # an instance of it def __repr__(self): return f"({self.name}, {self.age})" def set_parts(self, tails=1, ears=2, paws=4): self.parts = { # this sets the instance variable "tails": tails, "ears": ears, "paws": paws } return self.parts # You can also return the value like a standard function def sheds(self): if self.furry and not self.hypoallergenic: # using the instance variables within logic return True return False @static_method # this is a method that can be def bark(): # called without instantiating a class print('Bark Bark!') if __name__=="__main__": # Only run if the file is being explicitly ran as a script. # Here is how to use the class murphy = Dog("Murphy", 7) murphy.bark() murphy.set_parts() # ticks all of the boxes for the default if murphy.sheds(): print("You should buy a roomba.") champion = Dog("Champion", 2) champion.set_parts(1, 2, 3) # The dog from Parks and Rec only has 3 legs print(murphy) print(champion) # you can also call static methods like this Dog.bark()

Instance variables can be defined in functions with self.<var name>, and I prefer to define all the ones that I will be using in the __init__ method for my own organization. Class variables are defined outside of methods and are shared between all functions, even when changed. For example, If I were to change Murphy’s genus to ‘cat’ or something absurd, Champions’ genus would change as well. Also, classes can inherit other classes. Here is an example, assuming that the previous class-related code was imported previously.

from dogs import Dog class Labrador(Dog): def is_purebred(self): return True if __name__=='__main__': # the class labrador would have the same # methods, class vars and instance vars as # the previous dog class. marley = Labrador("Marley", 14) marley.bark() marley.set_parts() if marley.sheds(): print("Labs shed a lot.") if marley.is_purebred(): print("This dog is pure!")

In this example, I imported some of the previously written code. When you call the import statement, Python searches in a few places for your code. The places that it searches are called the PYTHONPATH, and your current directory is included. When you import another file in Python, the filename is omitted. Here is another import example:

import os from dogs import Dog name = os.getenv('DOG_NAME') age = os.getenv('DOG_AGE') dog = Dog(name, age)

In this import example, the standard library module os was used. This module gives access to operating system functions while remaining platform agnostic, so functions will work on any OS.

The order for imports, while not enforced, usually goes like this:

import standard_library import pypi_module import local_module

To install a PyPI module, run the command pip install <name>. Common modules include requests, a really simple HTTP REST request library, Flask, a popular and lightweight HTTP server framework, and Pillow, an image manipulation library. Many, many more exist, and can be found on PyPI.

Usually, when working with many Python packages, virtual environments are used. These make a project-level installation of packages, similar to how npm installs packages locally. See here for install instructions.

To use on Windows:

python -m venv env .\env\Scripts\activate

To use on MacOS & Linux:

python3 -m venv env source env/bin/activate

Then you can install packages as previously stated, but they will install to the virtual environment. To save all of the packages for later use on another system, run pip freeze > requirements.txt. To install packages from a fresh virtual environment:

python3 -m venv env source env/bin/activate pip install -r requirements.txt

All packages in requirements.txt will be installed to the virtual environment. This is equivalent Node’s package-lock.json file and should be committed to your source control.

This concludes Python For Programmers part 3.


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