Python Miscellaneous Syntax

1. Comprehensions

A compact, one-line way to create a list, set, or dictionary from a loop.

List Comprehension

# Traditional way
lst = []
for i in range(1, 11):
    lst.append(i)

# List comprehension — same result
lst = [i for i in range(1, 11)]        # → [1, 2, 3, ..., 10]

With an expression

lst = [i / 2 for i in range(1, 11)]   # → [0.5, 1.0, 1.5, ...]

With a condition (filter)

lst = [i / 2 for i in range(1, 11) if i % 2 == 0]   # only even numbers

With a nested loop

lst = [i * j for i in range(1, 11) for j in range(5)]

Set Comprehension

Use curly braces {} — duplicates are automatically removed.

s = {i for i in range(1, 11)}    # → {1, 2, 3, ..., 10}

Dictionary Comprehension

Use curly braces with a key: value pair.

d = {i: j for i in range(1, 11) for j in range(5)}

⚠️ Tuple Comprehension Doesn't Exist

Using () creates a generator object, not a tuple.

(i for i in range(10))    # → generator object (NOT a tuple)

# To get a tuple, use list comprehension then convert:
tuple([i for i in range(10)])

2. Multiple Assignment

Assign the same value to multiple variables in one line.

x = y = 1         # both x and y equal 1
x = y = z = 2     # x, y, and z all equal 2

3. Unpacking

Unpack an iterable (tuple, list) into individual variables. The number of variables must match the number of elements.

x, y = 1, 2            # x = 1, y = 2  (right side is a tuple)
x, y, z = 1, 2, 3      # x = 1, y = 2, z = 3

x, y = [10, 20]        # works with lists too

This is the same mechanism used when a function returns multiple values — the returned tuple gets unpacked.

4. Doc Strings

A doc string is a description comment placed at the top of a function (or class), written as a triple-quoted string.

def foo():
    """This is the foo function."""
    pass

def add(x, y):
    """
    Returns the sum of x and y.
    Both x and y should be numbers.
    """
    return x + y

help() — Read a Doc String

help(foo)    # prints the doc string of foo
help(len)    # → "Return the number of items in a container."
help(max)    # → full documentation for max()

help() works on any function, class, or data type. Press Ctrl+C to stop if output is very long.

Cheat Sheet

# List comprehension
[i for i in range(1, 11)]
[i * 2 for i in range(1, 11)]
[i for i in range(1, 11) if i % 2 == 0]
[i * j for i in range(5) for j in range(5)]

# Set comprehension
{i for i in range(1, 11)}

# Dictionary comprehension
{i: i * 2 for i in range(1, 11)}

# Tuple comprehension — doesn't work, use this instead:
tuple([i for i in range(10)])

# Multiple assignment
x = y = z = 0

# Unpacking
x, y = 1, 2
x, y, z = [10, 20, 30]

# Doc string
def my_func():
    """Describe what this function does."""
    pass

help(my_func)