"variable, variable =" syntax in python?

Question

I'm just getting started in python, and either haven't read about this, or missed it, and I don't know what to search for to find my answer.

Playing around with the IMAP module I came across this line of code.

result, data = mail.search(None, "ALL")

What is happening with the two variables here? Is this a syntax that is used when methods return a certain way, or does it always work? Could someone either explain what's going on here, or point me to some documentation?

Answer 1

You can assign multiple variables in Python in one line: -

a, b, c = 1, 2, 3

Assigns three values 1, 2, 3 to a, b, c respectively.

Similarly you can assign values from a list to variables.

>>> li = [1, 2, 3]    
>>> a, b, c = li
>>> a
1
>>> b
2

This unpacks your list into 3 variables

Answer 2

This is multiple assignment: the variables result and data simultaneously get the new values returned from mail.search(none, ALL).

The expressions on the right-hand side are all evaluated first before any of the assignments take place. The right-hand side expressions are evaluated from the left to the right.

the multiple assignment documentation is here

http://docs.python.org/tutorial/introduction.html

Answer 3

This is a form of sequence unpacking. If the RHS is an iterable of length 2 (since you have 2 objects on the LHS), you can use it. e.g.:

a,b = (1, 2)  #The RHS here is a tuple, but it could be a list, generator, etc.
print a #1
print b #2

Python3 extends this in an interesting way to allow the RHS to have more values than the LHS:

a,b,*rest = range(30) 
print(a) #0
print(b) #1
print(rest == list(range(2,30))) #True