Source

PEP 572 Assignment Expression

tl;dr

• This is a proposal for creating a way to assign to variables within an expression using the notation NAME := expr.

• Simple example group = re.match(data).group(1) if re.match(data) else None can become
  group = matched.group(1) if matched := re.match(data) else None

• In comprehension list filtered_data = [f(x) for x in data if (y := f(x)) is not None]
  to filtered_data = [y for x in data if (y := f(x)) is not None]

• For iteration of mutable

# Compute partial sums in a list comprehension
total = 0
partial_sums = [total := total + v for v in values]
print("Total:", total)

• Fetching specific variables with any and all

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

if all((nonblank := line).strip() == '' for line in lines):
    print("All lines are blank")
else:
    print("First non-blank line:", nonblank)

• Another example with while

# Loop-and-a-half
while (command := input("> ")) != "quit":
    print("You entered:", command)

• Top level assignement is not valid, y := f(x) will not work

Afterword

Already feel the need for this feature while coding some big comprehension list and can’t wait to use is in 3.8.