How to unzip a list of pairs into two lists in Python

Split a list of (a, b) tuples into two separate lists by iterating with a for loop and appending each element to its own output list.

Easy Python 3.9+ Aug 9, 2026 Lists & loops 14 views 0 copies

lists tuples loops unpacking

Python code

19 lines

Python 3.9+

def unzip(pairs):
    """Split a list of (a, b) pairs into two separate lists."""
    if not pairs:
        return [], []
    
    firsts = []
    seconds = []
    for a, b in pairs:
        firsts.append(a)
        seconds.append(b)
    
    return firsts, seconds


if __name__ == "__main__":
    pairs = [(1, 'a'), (2, 'b'), (3, 'c')]
    numbers, letters = unzip(pairs)
    print(f"First: {numbers}")
    print(f"Second: {letters}")

Output

stdout

First: [1, 2, 3]
Second: ['a', 'b', 'c']

How it works

The for a, b in pairs loop unpacks each tuple into two variables, so you can append the first element to firsts and the second to seconds in every iteration. Returning [], [] early handles the empty-input case so the function always returns two lists. This manual loop is explicit and easy to read, making it a clear approach when you want full control over the splitting logic. The function returns a tuple of two lists, which can be assigned directly to two variables with numbers, letters = unzip(pairs).

Common mistakes

Forgetting to handle an empty input list, which would cause the loop to return `None, None` instead of two lists
Assuming the pairs are always tuples — the unpacking works with any two-element iterable, but fails if an element has a different length
Mutating the original pairs list while iterating, which can skip elements or cause unexpected behavior

Variations

Use `zip(*pairs)` to achieve the same result in one line: `nums, letters = zip(*pairs)` (returns tuples)
Use a list comprehension with `enumerate` if you need indexes along with the split values

Real-world use cases

Splitting a list of coordinate pairs into separate X and Y arrays before plotting charts.
Separating key-value pairs fetched from a database into parallel lists for column-based processing.
Dividing paired measurement data (e.g., time and value) into two columns for CSV export.

How to unzip a list of pairs into two lists in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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