What does colon equal (:=) in Python mean?

Question

What does the := operand mean, more specifically for Python?

Can someone explain how to read this snippet of code?

node := root, cost = 0
frontier := priority queue containing node only
explored := empty set

Answer 1

Updated answer

In the context of the question, we are dealing with pseudocode, but starting in Python 3.8, := is actually a valid operator that allows for assignment of variables within expressions:

# Handle a matched regex
if (match := pattern.search(data)) is not None:
    # Do something with match

# A loop that can't be trivially rewritten using 2-arg iter()
while chunk := file.read(8192):
   process(chunk)

# Reuse a value that's expensive to compute
[y := f(x), y**2, y**3]

# Share a subexpression between a comprehension filter clause and its output
filtered_data = [y for x in data if (y := f(x)) is not None]

See PEP 572 for more details.

Original Answer

What you have found is pseudocode

Pseudocode is an informal high-level description of the operating principle of a computer program or other algorithm.

:= is actually the assignment operator. In Python this is simply =.

To translate this pseudocode into Python you would need to know the data structures being referenced, and a bit more of the algorithm implementation.

Some notes about psuedocode:

• := is the assignment operator or = in Python
• = is the equality operator or == in Python
• There are certain styles, and your mileage may vary:

Pascal-style

procedure fizzbuzz
For i := 1 to 100 do
    set print_number to true;
    If i is divisible by 3 then
        print "Fizz";
        set print_number to false;
    If i is divisible by 5 then
        print "Buzz";
        set print_number to false;
    If print_number, print i;
    print a newline;
end

C-style

void function fizzbuzz
For (i = 1; i <= 100; i++) {
    set print_number to true;
    If i is divisible by 3
        print "Fizz";
        set print_number to false;
    If i is divisible by 5
        print "Buzz";
        set print_number to false;
    If print_number, print i;
    print a newline;
}

Note the differences in brace usage and assignment operator.