Python Loops and Sequences

Source: freeCodeCamp — Learn Python (v9), Loops and Sequences module Companion to: python-basics-review

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Lists

Ordered, mutable, zero-based indexed. Can hold mixed types.

cities = ['Los Angeles', 'London', 'Tokyo']
cities[0]   # 'Los Angeles'
cities[-1]  # 'Tokyo'  (negative indexing from end)
len(cities) # 3

Create: list('Jessica') → ['J','e','s','s','i','c','a']

Modify:

lst[0] = 'new value'   # update by index
del lst[1]             # remove by index

Check membership: 'Rust' in lst → True/False

Unpack:

name, age, job = ['Alice', 34, 'Dev']  # exact count required
name, *rest   = ['Alice', 34, 'Dev']  # collect remainder with *

Slice: lst[start:stop:step] — stop is non-inclusive

desserts[1:3]   # ['Cookies', 'Ice Cream']
numbers[1::2]   # every other element starting at index 1

Nest: developer[2][1] — chain indices to access nested lists

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List Methods

sorted(lst) — returns a new sorted list, original unchanged.

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Tuples

Ordered, immutable, zero-based indexed. Use () or tuple().

dev = ('Alice', 34, 'Rust Developer')
dev[1]    # 34
dev[-2]   # 34

• Mutation raises TypeError; deletion raises TypeError
• Supports: indexing, negative indexing, slicing, in, unpacking, * rest collection — same as lists
• Use tuple when: the collection is fixed and should not change
• Use list when: you need to add, remove, or update elements

Tuple methods:

sorted(iterable, key=len, reverse=True) — optional key and reverse args work on any iterable.

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for Loop

Iterate over any sequence (list, tuple, string, range…).

for language in ['Rust', 'Java', 'Python']:
    print(language)

for char in 'code':
    print(char)

Nested loops: outer loop × inner loop iterations.

else clause: runs only if loop was NOT terminated by break.

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while Loop

Repeats while condition is True.

while guess != secret_number:
    guess = int(input('Guess: '))

Also supports else clause (executes if no break).

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break and continue

# break — exit loop immediately
for dev in names:
    if dev == 'Naomi': break
    print(dev)         # prints only up to (not including) Naomi

# continue — skip current iteration
for dev in names:
    if dev == 'Naomi': continue
    print(dev)         # prints all except Naomi

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range()

range(stop)            # 0 … stop-1
range(start, stop)     # start … stop-1
range(start, stop, step)

range(2, 11, 2)        # 2, 4, 6, 8, 10
range(40, 0, -10)      # 40, 30, 20, 10  (decrement)
list(range(2, 11, 2))  # [2, 4, 6, 8, 10]

• stop is non-inclusive
• Default start=0, default step=1
• Only accepts integers (float → TypeError); no args → TypeError

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enumerate() and zip()

# enumerate — index + value pairs
for i, lang in enumerate(languages):          # starts at 0
for i, lang in enumerate(languages, start=1): # custom start

list(enumerate(languages))
# [(0, 'Spanish'), (1, 'English'), ...]

# zip — parallel iteration over multiple iterables
for name, id in zip(developers, ids):
    print(name, id)
# stops at shortest iterable

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List Comprehensions

Create a new list in one line:

even = [num for num in range(21) if num % 2 == 0]
# [0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

Pattern: [expression for item in iterable if condition]

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Iterable Functions

# filter(func, iterable) — keep items where func returns True
long_words = list(filter(lambda w: len(w) > 4, words))

# map(func, iterable) — apply func to every item
fahrenheit = list(map(lambda t: (t * 9/5) + 32, celsius))

# sum(iterable, start=0)
sum([5, 10, 15, 20])       # 50
sum([5, 10, 15, 20], 10)   # 60

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Lambda Functions

Anonymous, single-expression functions. Most useful as arguments to filter(), map(), sorted().

filter(lambda x: x % 2 == 0, numbers)  # even numbers
sorted(langs, key=lambda s: len(s))     # sort by length

Best practices: don't assign to a variable; keep simple and short; use for one-off inline functions.