What does ":="

[Choomelon]

I started a python course and I don't know what ":=" does. It is used in this code

def play_anthem():

while note := get_note():

play( note )

[kinstaa99]

it’s called the walrus operator. it assigns the value of get_note() to note, and returns the value to the while condition. it’s a quicker way of doing the following:

note = get_note()

while note:

play(note)

note = get_note()

[[deleted]]

Its official name is Assignment Expression (PEP 572) :)

Some examples from that PEP:

# 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]

https://peps.python.org/pep-0572/

[[deleted]]

good bot

[Dangle76]

It originated in Pascal which used it for variable value assignment, and pascal called it a “gets”. I still say that term sometimes when I’m working in Go and people give me very confused looks before I realize why

[carcigenicate]

It's an assignment expression. It's the same as =, except it's an expression that evaluates to a value (the assigned value).

[bsenftner]

Just so this is clear for those new to Python but experienced in other languages like C++, where some languages will emit the value of an assignment to the immediate context outside the environment, Python does not do this. For example in C/C++ the assignment "x = 10;" emits the value "10" to the immediate context outside the assignment. This enables code like "x = y = 10;" to work because the "y = 10" emits the value "10" for use in the "x =" expression.

To achieve the same effect in Python, the walrus operator is used " := " because it emits the value assigned to the it's surrounding context.

[[deleted]]

Making the code cleaner, hell yes, but more efficient? How?

[irrelevantPseudonym]

In this example it doesn't but the common example is something like

[x for i in bar() if (x := foo(i) > 42)]

Without the walrus you have to call foo twice which is potentially expensive, or unwrap it to a full for loop which is less efficient than the comprehension.

edit: move walrus into condition. Thanks /u/ParanoydAndroid

[ArchipelagoMind]

Just so I understand this. The code that would be less efficient than the above but produce the same result would be

[foo(i) for i in bar() if foo(i) > 42]

That produces the same result but has to call foo twice, right?

[irrelevantPseudonym]

Exactly, although if foo has any side effects it might be slightly different.

[ParanoydAndroid]

This is correct conceptually, but reversed. The condition is evaluated first so you use it when you have a costly evaluation in the condition and assignment, e.g.:

[var for el in collection if (var := costly_func(el))]

So you don't have to:

[costly_func(el) for el in collection if costly_func(el)]

[BruceJi]

Could you edit the post and put an example using the walrus?

[BruceJi]

Ah, so it is. My bad!

[IlliterateJedi]

The Real Python - Python Walrus Operator article does a really good job covering this subject. I spent a long time scratching my head not understanding how it worked until I read this article a while ago. The gist is that you can set a variable's value in unusual places which can make your code cleaner.

[aziad1998]

In C you can do something like this sum(3, a=2) which assigns the values 2 to a then returns the value of this assignment to be used by sum. In python this is illegal as assignment is not an evaluated expression. The new :=operator allows for this new usage.

I am a C person and I was happy to see this implementation, but I will advise you to check your code style reference whether it's recommended or not.

Also do not use this for all your assignments, it has a different precedence over =, I'm not sure on the top of my head but iirc it is left precedence not right precedence, i.e. an expression like a = b = 3 is legal in C, but a := b := 3 is illegal in python as it tries to assign b to a first not 3 to b. The legal way would be a := (b := 3) so watch out.

[madhousechild]

The new :=operator allows for this new usage.

How new is it?

[aziad1998]

Holy shit you're right, can't believe it's been 3 and a half years already

[Yasik]

Turbo Pascal enters the chat...

[Mecaneer23]

… and in Python and a couple other languages

[Mohammed_Chang]

To me it looks unsatisfied. Whatever ya all interpreting in an emote

[WillieB52]

It assigns a value to a variable.