assignment.md

Pythonic assignment [Readability, Programming Efficiency]

Assigning the same value to multiple variables

This is a straightforward Python feature.

Not pythonic

a = 1
b = 1

Pythonic

a = b = 1

>>> print(a, b)
1 1
>>> b = 2
>>> print(a, b)
1 2

Conditional assignment

A pythonic way to assign values to a variable when the value depends on a condition, or when one variable has the possibility of being None, an empty list or empty string.

Not pythonic

a = 1
b = 2 

if a != b:
    x = "Not equal"
else:
    x = "Equal"

c = None
if c is None:
    c = "Value"

Pythonic

x = "Equal" if a == b else "Not equal"

>>> print(x)
"Not equal"

c = None
d = "Value"
y = c or d

>>> print(y)
"Value"

tuple and list unpacking

What is this?

Tuple and list elements can be accessed using indexes as in lists, however Python supports unpacking, a much cleaner approach.

Not pythonic

person = ("Aamir", 21) # also works if this is a list
name = person[0]
age = person[1]

Pythonic

This is not only more readable, but saves space as well.

name, age = person

catch-all unpacking

What is this?

Also known as asterisk unpacking, this is a sort of pattern matching where you can access some elements of a list or tuple individually, and store the "rest" in a list. The syntax is similar to tuple unpacking. This is also a much cleaner approach as opposed to, for example, list slicing.

Not pythonic

some_numbers = [1, 2, 3, 4, 5]
fst = some_numbers[0]
# 1
snd = some_numbers[1]
# 2
rest = some_numbers[2:]
# [3, 4, 5]

Pythonic

fst, snd, *rest = some_numbers
print(fst, snd, *rest)
# 1 2 [3, 4, 5]

fst, *mid, last = some_numbers
print(fst, *mid, last)
# 1 [2, 3, 4] 5

ignored variables

What is this?

When unpacking a list or tuple, you may want to ignore certain elements and not store then. In Python, this is what the _ variable is for. This variable is generally used as an "ignored" variable.

Example

fst, _, trd = (1, 2, 3)
# 2 is ignored 

the_list = [(1,2,3), (4,5,6), (7.8,9)]
for x, _, z in the_list:
    print(x, z)

Swapping two variables

Rather than using a temporary variable to swap values, Python has a simpler syntax to achieve the same result.

Not pythonic

a = 1
b = 2

# Swapping
temp = a
b = a
a = temp

Pythonic

a, b = b, a

Note that this can also work with more than 2 variables.

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References

[1] PEP 308 -- Conditional Expressions [2] Python docs -- unpacking
[3] PEP 3132 -- Extended Iterable Unpacking

Books that mention this topic

[4] Python Tricks: A Buffet of Awesome Python Features by Dan Bader
[5] Effective Python: 90 Specific Ways to Write Better Python by Brett Slatkin
[6] Python Cookbook, Third Edition by David Beazley and Brian K. Jones
[7] Writing Idiomatic Python 3 by Jeff Knupp
[8] The Little Book of Python Anti-Patterns by QuantifiedCode