How to Create a Line-Numbered Generator with enumerate start in Python

This Python code defines a generator that yields lines prefixed with their index, using enumerate's start parameter to offset numbering.

Easy Python 3.9+ Aug 9, 2026 Comprehensions & generators 14 views 0 copies

enumerate generator yield iteration

Python code

9 lines

Python 3.9+

def line_numbered_lines(lines, start=1):
    for idx, line in enumerate(lines, start):
        yield f"{idx:3} {line}"


if __name__ == "__main__":
    sample = ["first line", "second", "third"]
    for numbered in line_numbered_lines(sample, start=10):
        print(numbered)

Output

stdout

10 first line
 11 second
 12 third

How it works

The enumerate built-in returns pairs of an index and an element from an iterable. Passing start=10 makes the index begin at 10 instead of 0. The generator yields formatted strings with f"{idx:3} {line}", which right-aligns the index in a 3-character field. Using a generator means each line is produced lazily, so it works with large or infinite iterables without loading them into memory.

Common mistakes

Forgetting to pass `start` if you want numbering to start at 1; default is 0.
Using `yield` inside a regular function without the `yield` statement, causing the function to return a generator object unexpectedly.
Assuming enumeration works only on lists — it works on any iterable such as files or generators.

Variations

Use a generator expression with `enumerate` and `map` to produce the formatted lines: `(f"{i:3} {line}" for i, line in enumerate(lines, 1))`
Return a list comprehension if you need all lines at once, but be cautious with memory for large inputs.

Real-world use cases

Adding line numbers to error messages when linting or compiling source code, aiding developers in locating issues.
Printing numbered steps in a CLI tool for a checklist or setup wizard.
Generating formatted numbered lists for markdown reports or documentation from a loop of items.

How to Create a Line-Numbered Generator with enumerate start in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Common mistakes

Variations

Real-world use cases

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