for Loops, range() and Iterating Collections

Python’s for loop is different from JavaScript’s for loop in a fundamental way — it is a for-each loop that iterates over any iterable object directly, rather than incrementing an index variable. This makes Python loops more readable and less error-prone: there is no off-by-one error, no need to track an index variable, and no need to call .length. Python’s for loop works seamlessly with lists, tuples, strings, dictionaries, sets, generators, and any object that implements the iterator protocol. You will use for loops constantly in FastAPI to process database query results, transform API response data, and validate collections of values.

Basic for Loop #

# Iterate directly over items — no index needed
fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)
# apple
# banana
# cherry

# Iterate over a string (character by character)
for char in "hello":
    print(char)   # h, e, l, l, o

# Iterate over a range of numbers
for i in range(5):         # 0, 1, 2, 3, 4
    print(i)

for i in range(1, 6):      # 1, 2, 3, 4, 5
    print(i)

for i in range(0, 10, 2):  # 0, 2, 4, 6, 8 (step=2)
    print(i)

for i in range(10, 0, -1): # 10, 9, 8, ... 1 (countdown)
    print(i)

enumerate() — Index and Value Together #

posts = ["First Post", "Second Post", "Third Post"]

# Old way — verbose and error-prone
for i in range(len(posts)):
    print(f"{i}: {posts[i]}")

# Pythonic way — use enumerate()
for i, post in enumerate(posts):
    print(f"{i}: {post}")
# 0: First Post
# 1: Second Post
# 2: Third Post

# Start index at 1 (for human-readable numbering)
for i, post in enumerate(posts, start=1):
    print(f"Post {i}: {post}")
# Post 1: First Post
# Post 2: Second Post
# Post 3: Third Post

# Practical FastAPI use: add position to response items
results = db.get_posts()
numbered = [{"rank": i, "post": p} for i, p in enumerate(results, start=1)]

zip() — Looping Over Multiple Sequences #

names  = ["Alice", "Bob", "Charlie"]
scores = [95, 87, 92]
grades = ["A", "B+", "A-"]

# Loop over multiple lists in parallel
for name, score, grade in zip(names, scores, grades):
    print(f"{name}: {score} ({grade})")
# Alice: 95 (A)
# Bob: 87 (B+)
# Charlie: 92 (A-)

# zip() stops at the shortest sequence
a = [1, 2, 3, 4, 5]
b = ["a", "b", "c"]
list(zip(a, b))   # [(1, 'a'), (2, 'b'), (3, 'c')] — stops at 3

# Create a dict from two lists
keys   = ["name", "age", "role"]
values = ["Alice", 30, "admin"]
user   = dict(zip(keys, values))
# {"name": "Alice", "age": 30, "role": "admin"}

Iterating Over Dictionaries #

user = {"name": "Alice", "age": 30, "role": "admin"}

# Iterate over keys (default)
for key in user:
    print(key)           # name, age, role

# Iterate over values
for value in user.values():
    print(value)         # Alice, 30, admin

# Iterate over key-value pairs
for key, value in user.items():
    print(f"{key}: {value}")
# name: Alice
# age: 30
# role: admin

# Practical use: filter a response dict
def sanitise_user(user_dict: dict) -> dict:
    excluded = {"password", "password_hash", "reset_token"}
    return {k: v for k, v in user_dict.items() if k not in excluded}
# Removes sensitive fields before sending to the client

Common Mistakes #

Mistake 1 — Modifying a list while iterating over it #

❌ Wrong — removes items during iteration, skips some:

items = [1, 2, 3, 4, 5]
for item in items:
    if item % 2 == 0:
        items.remove(item)   # Bug! Skips items as list shrinks
print(items)   # [1, 3, 5] — looks correct but skipped item 4 in some cases

✅ Correct — use a list comprehension to filter:

items = [1, 2, 3, 4, 5]
items = [item for item in items if item % 2 != 0]
print(items)   # [1, 3, 5] ✓

Mistake 2 — Using range(len()) when enumerate() is cleaner #

❌ Wrong — JavaScript-style index loop:

for i in range(len(posts)):
    print(f"{i}: {posts[i]}")   # verbose and error-prone

✅ Correct — enumerate():

for i, post in enumerate(posts):
    print(f"{i}: {post}")   # ✓ cleaner

Mistake 3 — Forgetting range() is exclusive of the end #

❌ Wrong — expecting range(1, 5) to include 5:

for i in range(1, 5):
    print(i)   # 1, 2, 3, 4 — NOT 5!

✅ Correct — range end is exclusive, use range(1, 6) to include 5:

for i in range(1, 6):
    print(i)   # 1, 2, 3, 4, 5 ✓

Quick Reference #