Querying container size

Obtain the length of a list, tuple, string, dict, set, or any object implementing the Sized protocol.

Data analysis: determining the number of rows in a list before processing; Web development: validating that a request payload does not exceed a maximum length

General Python scripts, data‑processing pipelines, algorithm implementations, web back‑ends.

len(iterable)

len(...)

iterable: any object that implements the Sized protocol (e.g., list, tuple, dict, string, set, custom class)

• list
• tuple
• dict
• string
• set
• range
• custom collection classes

• obj.__len__()
• sum(1 for _ in obj) for iterables without __len__
• len(list(obj)) after materialising an iterator

Calling len on a generator or iterator (TypeError), forgetting the parentheses (len vs len), using len on an integer, assuming O(N) cost for custom lenlenlen implementations.

Using obj.__len__() directly bypasses the built‑in, while size() is common in C++ STL containers; len() is Python‑specific.

Coming from C/C++: confusing len() with sizeof, which yields byte size at compile time.

• built‑in functions
• collection utilities
• sequence protocols

len() runs in O(1) for built‑in containers but may be O(N) for user‑defined __len__ methods; avoid calling len on large lazy iterables without materialising them.

Enables constant‑time size checks, simplifies loops and bounds validation, and makes code intent explicit.

Think of "len" as the shortcut for "length" of a collection.

len() is a built‑in function that invokes the object's __len__ method; it cannot be used directly on generators or iterators lacking __len__ (use sum(1 for _ in gen) instead).

Use collections.abc.Sized to type‑check objects or operator.length_hint for iterables lacking __len__.