What is Python Assignment Expressions 3.8?

Question

As assignment Expressions are defined in the PEP 572 which has been approved to be implemented in version 3.8 of Python.

• But what are the assignment Expressions and when they should be used?
• What kind of problem your implementation seeks to solve?
• Why has its implementation caused controversy in the Python community? It goes against the "principles" of the language?

Answer 1

The answer goes by levels.

• Simple concept:
• Complete concept:
• Example:
• How to use:
• References:

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Simple concept:

We can say in a way " "simple" that is to define a variable and pass it as value at the same time.

Ex.:

# $ entrada do código 
# > saída do código 

$ a = 1
>
$ a
> 1

So what you can do is put the two things together in a single line. Getting like this:

$ a := 1    # Você definiu a variável e ao mesmo tempo ela passa o valor adiante.
> 1

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Complete concept:

Abstract:

It is a proposal to define variables as an expression. ( NAME := expr )

• added a new exception "Targetscopeerror".

Motivation:

Naming the result of an expression is an important part of programming, thus allowing it to be used in place of a long expression and allowing reuse.

...
a = foo(x)
baa(a)
cee(a)
...

Now it may be :

...
baa(a:=foo(x))
cee(a)
...

The real importance:

I will not quote directly here because it is great and it is good to read everything, the link is at the end of the answer. If anyone wants I summarize and change here. And yes had discussions if this is pythonic.

Syntax:

Many arbitrary contexts in which Python expressions can be used. This is the form of NAME := expr where expr is a valid Python expression as a tuple without parentheses and NAME is the identifier.

$ a = 3,1
$ type(a)
> <class 'tuple'>

regex

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

Loop cannot be easily rewritten using 2 iterative arguments.

while chunk := file.read(8192):
   process(chunk)

Analyzing value is computationally expensive

[y := f(x), y**2, y**3]

Share the sub-expression between a filter and an "output".

filtered_data = [y for x in data if (y := f(x)) is not None]

Exceptional cases: (Invalid and valid but not recommended.)

Tomorrow put calmly, because of my schedule, but has the link reference PEP 572 at the end of the answer. : ) If to use recommend reading.

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Example:

If anyone wants, I create a more complex example, but I will leave one that I found very simple but demonstrative that is in the PEP itself.

# Computar uma lista de somas parciais e retornar o total
total = 0
partial_sums = [total := total + v for v in values]
print("Total:", total)

# Legal né :)

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Reference:

PEP 572 -- Assignment Expressions

Example of use - VIDEO - EN

Sorry for the mistakes, whatever I can improve, just say in the comments.

See you around :)