Pythonic Idioms Beginner¶

Unpacking¶

Assign multiple names from a sequence at once, swap without a temp, or capture "the rest" with *. Cleaner than index-by-index access.

# Tuple unpacking
a, b, c = 1, 2, 3

# Swap without temp variable
a, b = b, a

# Star unpacking
first, *rest = [1, 2, 3, 4, 5]
# first = 1, rest = [2, 3, 4, 5]

*head, last = [1, 2, 3, 4, 5]
# head = [1, 2, 3, 4], last = 5

EAFP over LBYL¶

Pythonic style: try the operation and handle the exception, rather than pre-checking. Avoids race conditions and is often clearer — but dict.get is cleaner still for defaults.

# Bad — Look Before You Leap (LBYL)
if "key" in dictionary:
    value = dictionary["key"]

# Good — Easier to Ask Forgiveness than Permission (EAFP)
try:
    value = dictionary["key"]
except KeyError:
    value = "default"

# Even better
value = dictionary.get("key", "default")

Use enumerate, not range(len())¶

When you need both index and value, enumerate is clearer and less error-prone than indexing. Pass start=1 for human-friendly numbering.

# Bad
for i in range(len(items)):
    print(i, items[i])

# Good
for i, item in enumerate(items):
    print(i, item)

# Start from 1
for i, item in enumerate(items, start=1):
    print(f"{i}. {item}")

Use zip to iterate in parallel¶

Walk two or more sequences together without index bookkeeping — pairing names with scores, keys with values, etc.

# Bad
for i in range(len(names)):
    print(names[i], scores[i])

# Good
for name, score in zip(names, scores):
    print(name, score)

Truthiness checks¶

Empty containers and 0/""/None are falsy — test them directly. Use is None for None and is True for identity, not ==.

# Bad
if len(my_list) > 0:
if my_string != "":
if value == None:
if value == True:

# Good
if my_list:
if my_string:
if value is None:
if value is True:

One-liner patterns¶

A toolkit of concise expressions — ternaries, dict.get defaults, join, any/all — that replace multi-line boilerplate when the logic is simple.

# Conditional expression
label = "even" if x % 2 == 0 else "odd"

# Dictionary with get
name = config.get("name", "Anonymous")

# Join strings
", ".join(["apple", "banana", "cherry"])   # "apple, banana, cherry"

# any() and all()
has_negative = any(x < 0 for x in numbers)
all_positive = all(x > 0 for x in numbers)

The Zen of Python¶

Python's guiding design aphorisms (PEP 20). Run import this anytime you need a reminder of what "Pythonic" means.

import this

Key principles to remember:

• Beautiful is better than ugly
• Explicit is better than implicit
• Simple is better than complex
• Readability counts
• There should be one obvious way to do it

Practice exercises¶

1. Rewrite a block of code using unpacking instead of index access.
2. Convert five if key in dict checks to use .get().
3. Rewrite a range(len()) loop to use enumerate.
4. Use any() and all() to validate a list of inputs.