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

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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

What you have found is pseudocode

http://en.wikipedia.org/wiki/Pseudocode

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

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

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

Note that there are certain styles of pseudocode and your mileage may vary:

Pascal Style PseudoCode
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 Pseudocode
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.

What does colon equal (:=) in Python mean?, What you have found is pseudocode. http://en.wikipedia.org/wiki/Pseudocode. Pseudocode is an informal high-level description of the  In math, the colon-equals is meant for definitions, and is thus inherently global in nature; if you write "j := 0", you are making the statement that henceforth, "for all time", the symbol "j" will be synonymous with the symbol "0".

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

This syntax will be available in Python 3.8.

Python Programming/Sequences, : allows the square brackets to take as many as two numbers. For any sequence which only uses numeric indexes, this will return the portion which is between the specified indexes. This is known as "slicing," and the result of slicing a string is often called a "substring." Colons (:) are basic and important in python. Other languages use { } or other method to indicate/use a code block. Python use indentation after colons (:) to indicate/use a code block. Every time group of statements follows, that line ends in colon and the block is indented.

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]

What does a double colon followed by an equals sign (::=) mean in , language documentation you refer to, an identifier is defined as a letter or an underscore, followed by a letter, a digit or an underscore. In python this is simply the = operator. 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

Happy 3.8 Release on 14th of October

There is new syntax := that assigns values to variables as part of a larger expression. It is affectionately known as "the walrus operator" due to its resemblance to the eyes and tusks of a walrus.

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)")

https://docs.python.org/3/whatsnew/3.8.html

What does "**" mean in python?, '**' takes a dict and extracts its contents and passes them as parameters to a function. Take this function for example: def func(a=1, b=2, c=3): print a print b print b. just remove :str and it will work in python 2 (this is a new python 3.5 feature to check type) – Jean-François Fabre ♦ Jan 14 '17 at 8:59 @TigerhawkT3: this is duplicate party today :) hopefully I'll be able to dupehammer questions in a few weeks. working on it.

What does colon ':' operator do in Python?, The : operator slices a part from a sequence object such as list, tuple or string. It takes two arguments. First is the index of start of slice and second  So to sum it up, the colon equal combination := is used to set the value of a parameter (argument) for a property or method. The equal sign is use to set the value of a property, object, variable, or as a comparison operator.

PEP 572 -- Assignment Expressions, Additionally, naming sub-parts of a large expression can assist an interactive A lambda (being an explicit, if anonymous, function definition) counts as a Has the C problem that an equals sign in an expression can now  If the value of left operand is greater than or equal to the value of right operand, then condition becomes true. (a >= b) is not true. <= If the value of left operand is less than or equal to the value of right operand, then condition becomes true. (a <= b) is true.

6. Expressions, This chapter explains the meaning of the elements of expressions in Python. needs two expressions separated with a colon followed by the usual “for” and “if” The arguments may be floating point numbers, e.g., 3.14%0.7 equals 0.34  The colon ( : ) is a punctuation mark consisting of two equally sized dots centered on the same vertical line. A colon precedes an explanation or an enumeration, or list. A colon is also used with ratios, titles and subtitles of books, city and publisher in bibliographies, biblical citations between chapter

Comments
  • This is not Python code; this is more a (formal) syntax declaration, e.g. of some configuration file. Compare docs.python.org/3/library/string.html#formatstrings for example.
  • := means Syntax Error in Python, e.g. it is not valid syntax when used in Python code.
  • Or pseudocode. := usually means assignment, though.
  • this is a pretty valid question from someone who is just learning python and just found some (and didn't realize it) pseudocode. I don't think this needs downvotes, just a straight forward answer.
  • Yeah, sorry, I'm really new to python and was trying to make sense of some random code I found. Thanks for the clarification on it being pseudo code.
  • Hi @MikeMcMahon, just a heads up -- as of the introduction of assignment operator syntax, which uses :=, in PEP572 (due to be implemented for Python 3.8), this answer is out-of-date.
  • @Jules in the context of the question it is very accurate as := would bomb out on the examples, but a link to the relevant PEP is likely in order.
  • @JulienChien How does that relate?
  • @MaxRied: how does it not, if it's in an accepted PEP and is gonna be part of Python after 3.8 release?
  • @z33k The question was asked years before the PEP was created. This certainly doesn't answer the question. Could be a comment though...
  • PEP 577 is in competition with PEP 572.
  • It was; it was withdrawn by its author.
  • Oh, you're right. That's too bad. I liked 577 better.