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Control Flow

Enumerate

Getting index and value in loops

Interview: Tests knowledge of Pythonic idioms — interviewers look for enumerate over range(len())

Last Updated: June 12, 2026 • 8 min read

The enumerate() built-in adds a counter to any iterable and returns an iterator of (index, value) tuples. It's the Pythonic way to get both the index and value when iterating, replacing the anti-pattern of range(len()).

Basic Usage

  • Signature: enumerate(iterable, start=0)
  • Returns: An iterator (lazy, memory-efficient) yielding (index, element) tuples
  • Works with any iterable: Lists, strings, dicts, files, generators, custom iterables
  • start parameter: Begin counting from any value (common use: 1-based indexing)

Why enumerate() Over range(len())

  • More readable: Intent is clear — you want index + value
  • Works with generators: range(len()) fails on generators (no len)
  • No double indexing: lst[i] inside the loop is unnecessary and error-prone
  • Works with non-sequence iterables: Files, sets, dict keys, etc.

Advanced Patterns

  • Unpacking in comprehension: {val: i for i, val in enumerate(items)} — creates value-to-index mapping
  • Parallel with zip: for i, (a, b) in enumerate(zip(list1, list2)):
  • Filtering with index: [v for i, v in enumerate(data) if i % 2 == 0] — keep even-indexed elements
  • Building dicts: dict(enumerate(items)) — creates index-to-value mapping
  • Progress tracking: Use enumerate to report progress in long-running loops

Interview Tip

If you catch yourself writing for i in range(len(lst)) and then using lst[i], immediately refactor to enumerate(). This is one of the most common Pythonic code review suggestions.

Under the Hood

enumerate() is implemented in C and is very fast. It's equivalent to this Python generator, but faster:

  • It creates an iterator object that wraps the original iterable
  • Each call to next() returns the next (counter, value) tuple
  • Memory usage is O(1) regardless of the iterable size (it's lazy)

Use Cases

Displaying numbered lists to users (with start=1)

Building index-based mappings from sequences

Filtering elements based on their position

Tracking progress in data processing pipelines

Common Mistakes

Using range(len(lst)) with lst[i] instead of enumerate — less readable and fails for generators

Forgetting to unpack the tuple: writing enumerate(lst) without i, val in the for loop

Using enumerate when you only need values (just iterate directly: for val in lst)

Not knowing enumerate works with any iterable (files, generators), not just lists