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

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 cant be trivially rewritten using 2-arg iter()
while chunk := file.read(8192):
   process(chunk)

# Reuse a value thats 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.

PEP572 proposed support for the := operator in Python to allow variable assignments within expressions.

This syntax is available in Python 3.8.

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

The code in the question is pseudo-code; there, := represents assignment.

For future visitors, though, the following might be more relevant: the next version of Python (3.8) will gain a new operator, :=, allowing assignment expressions (details, motivating examples, and discussion can be found in PEP 572, which was provisionally accepted in late June 2018).

With this new operator, you can write things like these:

if (m := re.search(pat, s)):
    print m.span()
else if (m := re.search(pat2, s):
    …

while len(bytes := x.read()) > 0:
    … do something with `bytes`

[stripped for l in lines if len(stripped := l.strip()) > 0]

instead of these:

m = re.search(pat, s)
if m:
    print m.span()
else:
    m = re.search(pat2, s)
    if m:
        …

while True:
    bytes = x.read()
    if len(bytes) <= 0:
        return
    … do something with `bytes`

[l for l in (l.stripped() for l in lines) if len(l) > 0]