What is the functools.reduce function?

Question

Accepted Answer

**reduce** applies a function **cumulatively** to reduce a sequence to a single value.

```python
from functools import reduce

# Sum of list
result = reduce(lambda acc, x: acc + x, [1, 2, 3, 4, 5])
print(result)  # 15

# With initial value
result = reduce(lambda acc, x: acc + x, [1, 2, 3], 10)
print(result)  # 16

# Find maximum
max_val = reduce(lambda a, b: a if a > b else b, [3, 1, 4, 1, 5, 9])
print(max_val)  # 9

# Flatten nested list
nested = [[1, 2], [3, 4], [5, 6]]
flat = reduce(lambda acc, x: acc + x, nested)
print(flat)  # [1, 2, 3, 4, 5, 6]

# Build dict from pairs
pairs = [("a", 1), ("b", 2), ("c", 3)]
result = reduce(lambda d, pair: {**d, pair[0]: pair[1]}, pairs, {})
print(result)  # {"a": 1, "b": 2, "c": 3}

# Pipe functions
from functools import reduce

functions = [
    lambda x: x + 1,
    lambda x: x * 2,
    lambda x: x - 3,
]

pipe = reduce(lambda f, g: lambda x: g(f(x)), functions)
print(pipe(5))  # ((5 + 1) * 2) - 3 = 9
```

**Use cases:**
- Aggregation.
- Function composition.
- Building complex data structures.

Python: What is the functools.reduce function?