Python - robbiehume/CS-Notes GitHub Wiki

Colab Notebooks

Links

General resource websites:

Look into

General Notes

  • Check if variable or attribute exists, without causing an error if it doesn't: 
    • if 'myVar' in locals():
    # myVar exists in local scope
if 'myVar' in globals():
    # myVar exists in global scope
if hasattr(obj, 'attr_name'):
    # obj.attr_name exists
  • Convert str to json, only if it exists
    • data = json.loads(body) if (body := event.get("body")) else body

Python Environment Notes

pip

  • Install pip: link
  • PyPI is the central repository of software that pip uses to download from
  • pip installs packages, which are usually bunded into 'wheels' (.whl) prior to installation
  • A wheel is a zip-style archive that contains all the files necessary for a typical package installation
  • It may be best to run it as python3 -m pip instead of using the system-installed pip
    • Why you should use python -m pip
    • This uses executes pip using the Python interpreter you specified as python
    • This is beneficial so you know what pip version is being used if you have multiple python versions installed

Tips

  • See package dependencies: pipdeptree
    • Install it first with pip install pipdeptree
  • Loop through n first elements in list link: for item in itertools.islice(my_list, n)

Wheel files (.whl)

Virtual environments (venv)

  • Complete guide to python virtual environments
  • Virtual environments are used when you want to be more explicit and limit packages to a specific project
  • You should never install stuff into your global Python interpreter when you develop locally
  • To create an environment: python -m venv

Version features

3.8

  • Assignment Expressions (link): can use := in an expression in a while loop or if statement to assign and evaluate it
    • Ex: if (y := 2) > 1: # sets y = 2 and evaluates the expression as 2 > 1
    • Ex: while (user_input := input("Enter text: ")) != "stop": # keeps getting user input until "stop" is entered
    • Can also use it in list comprehensions: [result for i in range(5) if (result := func(i)) == True]
      • It is more efficient because it potentially only makes half the func() calls compared to [func(i) for i in range(5) if func(i) == True]

3.9

  • Dictionary unions: d1 | d2 results in new dictionary resulting from the union of d1 and d2
    • Can use |= to do an in-place union: d1 |= d2 // will make d1 equal to the resulting union
  • String methods to remove prefix and suffixes: .removeprefix() and .removesuffix()

Python Language Notes

Variables and parameter passing

  • Deep dive into variables in Python
  • Python uses pass-by-object-reference for function parameter passing
    • Any changes to the actual object held in the parameter will change the original variable
    • Any reassignment will not be reflected in the original variable
  • When passing a variable of a mutable object (list, dict, set, classes, etc.), make sure that you pass a copy if you don't want the original variable object to be modified by any changes in the function

Dunder Methods (aka Magic Methods)

  • Dunder methods are methods that allow instances of a class to interact with the built-in functions and operators of the language

** (un)packing

Add to google colab:

  • String methods: .startswith() and .endswith() instead of string slicing
  • Create dictionary from two diffferent sequences / collections: dict(zip(list1, list2))
  • Update multiple key/value pairs in a dticitonary: d1.update(key1=val1, key2=val2) or d1.update({'key1': 'val1', 'key2': 'val2'})
    • Ex: class_grades.update(algebra='A', english='B')

Dictionaries:

Generators:

http, requests, & urllib(3) modules

http

requests

urllib3

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