Meaning
Returns the number of items in an object that implements __len__. It provides a quick way to check the size of collections without manual iteration. You reach for it whenever you need to know if a container is empty, bound a loop, or validate input length.
Primary Function
Length measurement
Communicative Purpose
Determines the number of elements in a collection.
Pattern
len(sequence)
Core Structure
len(...)
Função primária
Length measurement
Propósito comunicativo
Determines the number of elements in a collection.
Situações de gatilho
Data processing: checking if a list is empty before iteration; File handling: verifying the number of lines read from a file; String manipulation: validating that a username meets minimum length requirements.
Contextos
Python standard library, data analysis, web development, scripting.
Padrão
len(sequence)
Estrutura central
len(...)
Slots de substituição
sequence: any sized object (list, str, tuple, dict, etc.)
Colocados típicos
- Used with conditional statements (if len(seq) == 0)
- loops (for i in range(len(seq)))
- and slicing (seq[:len(seq)//2]).
Substituições comuns
- In NumPy
- prefer .size attribute
- in JavaScript
- use .length property
- manual counting loop as fallback.
Erros comuns
Calling len(None) raises TypeError: object of type 'NoneType' has no len(); Using len on a generator yields TypeError: object of type 'generator' has no len(); Confusing len with capacity (e.g., list capacity vs length).
Similar / contraste
len vs .size in NumPy arrays (.size gives total elements, len gives first dimension); len vs length property in JavaScript strings (both return character count).
Interferências
Coming from JavaScript: may try to access .length directly on Python lists — use len() function instead.
Família do chunk
- len
- sum
- max
- min
- any
- all
Nuance
Do not use on objects without __len__ (e.g., file streams, generators) as it raises TypeError; performance is O(1) for built-in types but may be O(n) for custom objects that compute length; works on any sized object and always returns a non‑negative integer.
Efeito pragmático
Enables constant‑time size checks, preventing off‑by‑one errors and making code clearer than manual counting loops.
Dica de memória
len is like peeking into a bag to see how many items are inside without emptying it.
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