functools.reduce
- Python course
- Advanced
- Lesson 5/83
- 5 min
Learn accumulation with functools.reduce.
functools.reduce
functools.reduce folds a sequence into a single value. It applies a two-argument function repeatedly: at each step it combines the accumulated value with the next item.
The syntax is reduce(fn, seq, initial). fn(accumulator, item) is called for each item.
from functools import reduce
nums = [1, 2, 3, 4]
total = reduce(lambda a, b: a + b, nums, 0)
print(total) # 10Without an initial value, the first item of the sequence is used. Providing an initial value is safer for empty sequences.
Product and chaining with reduce
reduce is not just for sums. Use lambda a, b: a * b for products; the initial value must be 1 (since 1 is the multiplicative identity).
The initial value matters: 0 for sums, 1 for products, "" for string concatenation. A wrong initial value corrupts the result.
Without an initial value, reduce on an empty sequence raises TypeError. Safe code always provides one.
Practice Compute a product with reduce
Compute the product of nums = [2, 3, 4] with reduce. Use lambda a, b: a * b as the combiner and 1 as the initial value. Print the result.
Starter code
from functools import reduce
nums = [2, 3, 4]
# use reduce with a multiplication combiner and initial 1
# print the resultSample solution
from functools import reduce
nums = [2, 3, 4]
result = reduce(lambda a, b: a * b, nums, 1)
print(result)Expected output
24
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the reduce call
from functools import reduce
nums = [1, 2, 3, 4]
total = ____(lambda a, b: a + b, nums, 0)
print(total)from functools import reduce
nums = [1, 2, 3, 4]
total = reduce(lambda a, b: a + b, nums, 0)
print(total)Explanation: reduce(lambda a, b: a+b, nums, 0) folds the list into a sum; the result is 10.
2 Match the reduce steps
| Term | Meaning |
|---|---|
fn(accumulator, item) |
Per-step call |
initial=0 |
Starting accumulator |
lambda a, b: a+b |
Summing combiner |
reduce |
Fold operation |
3 What should the reduce initial value be for a product?
- 1 Correct answer
- 0
- Empty string ""
- None
Explanation: 1 is the multiplicative identity; starting at 1 keeps the product correct.