What I've learned today - 2 - Walrus Operator (:=) in Python

What is?

The new operator allows variable assignments within expressions (also called Walrus Operator). It started as a proposal in PEP572 and became available in Python 3.8.

Benefits

• Make things more compact.
• Compress code when using regular expressions.
• Speed up the processing of large data.
• Code more readable.

Examples

A good example can be found in PEP572 documentation:

• Current:
  

  reductor = dispatch_table.get(cls)
  if reductor:
      rv = reductor(x)
  else:
      reductor = getattr(x, "__reduce_ex__", None)
      if reductor:
          rv = reductor(4)
      else:
          reductor = getattr(x, "__reduce__", None)
          if reductor:
              rv = reductor()
          else:
              raise Error(
                  "un(deep)copyable object of type %s" % cls)
  
  

• Improved:
  

  if reductor := dispatch_table.get(cls):
      rv = reductor(x)
  elif reductor := getattr(x, "__reduce_ex__", None):
      rv = reductor(4)
  elif reductor := getattr(x, "__reduce__", None):
      rv = reductor()
  else:
      raise Error("un(deep)copyable object of type %s" % cls)
  

This code is from python's core, and is a improved version of copy.py

We can use in any condition structure:

# While loop
while (command := input("> ")) != "quit":
    print("You entered:", command)

# Any
if any((comment := line).startswith('#') for line in lines):
    print("First comment:", comment)
else:
    print("There are no comments")

# List comprehenssion
stuff = [[(f(x) as .y), x/.y] for x in range(5)] # with "as"

Observations

The operator cannot be used for everything, in some cases he won't work:

a := 1 #INVALID must be done with a=1.
a = b := 2 #INVALID must be done with a=b=2

I appreciate everyone who has read through here, if you guys have anything to add, please leave a comment.