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:
stuff = [(lambda y: [y,x/y])(f(x)) for x in range(5)]
can become:
stuff = [[y := f(x), 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:
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
# 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)