What are assignment expressions (using the “walrus” or “:=” operator)? Why was this syntax added?

PEP 572 contains many of the details, especially for the first question. I’ll try to summarise/quote concisely arguably some of the most important parts of the PEP:

Rationale

Allowing this form of assignment within comprehensions, such as list comprehensions, and lambda functions where traditional assignments are forbidden. This can also facilitate interactive debugging without the need for code refactoring.

Recommended use-case examples

a) Getting conditional values

for example (in Python 3):

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

can become:

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

Similarly, from the docs:

In this example, the assignment expression helps avoid calling len()
twice:

if (n := len(a)) > 10:
    print(f"List is too long ({n} elements, expected <= 10)")

b) Simplifying list comprehensions

for example:

[(lambda y: [y, x/y])(x+1) for x in range(5)]

can become:

[[y := x+1, x/y] for x in range(5)]

Syntax and semantics

In any context where arbitrary Python expressions can be used, a named expression can appear. This is of the form name := expr where expr is any valid Python expression, and name is an identifier.

The value of such a named expression is the same as the incorporated expression, with the additional side-effect that the target is assigned that value

Differences from regular assignment statements

In addition to being an expression rather than statement, there are several differences mentioned in the PEP: expression assignments go right-to-left, have different priority around commas, and do not support:

• Multiple targets

  x = y = z = 0  # Equivalent: (z := (y := (x := 0)))
  

• Assignments not to a single name:

  # No equivalent
  a[i] = x
  self.rest = []
  

• Iterable packing/unpacking

  # Equivalent needs extra parentheses
  loc = x, y  # Use (loc := (x, y))
  info = name, phone, *rest  # Use (info := (name, phone, *rest))
  
  # No equivalent
  px, py, pz = position
  name, phone, email, *other_info = contact
  

• Inline type annotations:

  # Closest equivalent is "p: Optional[int]" as a separate declaration
  p: Optional[int] = None
  

• Augmented assignment is not supported:

  total += tax  # Equivalent: (total := total + tax)