10.2 Creating Lists

Empty Lists

The starting point for building up a collection incrementally, e.g. inside a loop.

>>> items = []
>>> items
[]

List Literals

The most common way to create a list with known contents up front — square brackets with comma-separated values.

>>> nums = [1, 2, 3]

list()

The list() constructor converts any iterable — string, tuple, range, set — into a list, covered in depth in 8.6 Collection Type Conversion.

>>> list((1, 2, 3))
[1, 2, 3]

Lists from Strings

Passing a string to list() splits it into its individual characters — a different result than str.split(), which splits by word or delimiter (see 9.7 Common String Methods).

>>> list("abc")
['a', 'b', 'c']

range()

range() alone is lazy — wrapping it in list() actually materializes the numbers as a real list (see 5.4 Sequence Data Types).

>>> list(range(5))
[0, 1, 2, 3, 4]

Nested Lists

A list can contain other lists as elements — the basis for representing grids, matrices, and tables; see 10.10 Nested Lists and Matrices.

>>> grid = [[1, 2], [3, 4]]

Mixed Data Types

Unlike arrays in many other languages, a single Python list can freely mix types.

>>> mixed = [1, "two", 3.0, True]

Quick Interview Answer

“A list literal ([1, 2, 3]) is the common case for known contents. list() converts any iterable — a string becomes its individual characters, not words, which trips people up if str.split() was actually intended. range() is lazy on its own; list(range(...)) is what actually materializes the numbers. Lists can nest freely and mix types, since every slot is just a same-sized reference regardless of what it points to.”

Common Mistakes

• Calling list("hello world") expecting a list of words — it produces a list of individual characters, including the space; str.split() is the word-splitting tool.
• Materializing a large range() into a list() unnecessarily, losing its lazy memory efficiency for no reason.
• Writing [[0] * 3] * 3 to build a 3×3 grid of independent rows — * on a list repeats references to the same inner list, so mutating one row mutates all of them (see 10.10 Nested Lists and Matrices for the correct pattern).