Size inspection

Report how many items are stored in a mapping

Data processing: checking if a dict has entries before processing; Loop control: limiting loops based on dict size; Validation: verifying input dict length before use

Python codebases, data processing scripts, API response handling, configuration parsing

len(container)

len(...)

container: mapping object (e.g., dict)

• checking if dict is empty with if len(d) > 0
• iterating over keys with for k in d
• retrieving values with d.get(k)

• len(my_list)
• len(my_set)
• len(my_string)

Using len on non-iterable objects like integers: cause: passing non-iterable to len; consequence: TypeError. Expecting len to count values instead of keys: cause: misunderstanding len behavior; consequence: incorrect size logic. Calling len on None: cause: passing None; consequence: TypeError.

my_dict.__len__() — explicit method call; len(list(my_dict.values())) — counts values (same as keys for dict but differs for other mappings); len(my_dict.keys()) — redundant call.

Coming from JavaScript: you might think to use Object.keys(my_dict).length → use len(my_dict); Coming from Java: you might think to use my_dict.size() → use len(my_dict).

• len
• sum
• any
• all

When NOT to use: when you need to count values or nested items, or when __len__ may be expensive; Performance: O(1) for built-in mapping types, depends on __len__ implementation for user-defined types; Boundary conditions: raises TypeError for non-sized objects, returns 0 for empty containers, does not count nested elements.

Provides instant, readable size check; avoids manual iteration and potential off-by-one errors.

Think 'length' → len(container).

len operates in O(1) time for built-in mapping types; for user-defined types performance depends on their __len__ implementation.

Using collections.Counter for frequency counts