reduce is suitable for aggregation operations such as accumulation, factorial calculation, etc.; the usage is to process iterable objects through accumulation function. 1. The function must accept two parameters and merge elements in sequence; 2. The optional initial value avoids empty list errors; 3. It is often used in arithmetic operations, string splicing and other scenarios; 4. Complex logic needs to be used with caution to ensure readability.
Using functools.reduce
for data aggregation is actually quite straightforward, the key is to understand how it works. It is not a function that can be understood at a glance, but once you know how to use it, it will be much simpler to write the code.

How does reduce work?
The basic function of reduce
is "cumulative calculation". It accepts a function and an iterable object (such as a list), and then "merges" the elements from left to right. This process is a bit like snowballing: the first element is processed together with the second element, the result is processed with the third element, and so on.
Its format is like this:

functools.reduce(function, iterable[, initializer])
-
function
is a function with two parameters. -
iterable
is the data you want to process. -
initializer
is the initial value (optional).
For a simple example, if you want to add up all the numbers in a list:
from functools import reduce nums = [1, 2, 3, 4] result = reduce(lambda x, y: xy, nums) print(result) # output 10
If there is no initial value, it starts with the first two elements by default. If you add the initial value, such as reduce(lambda x, y: xy, nums, 10)
, it will add from 10.

When should I use reduce?
reduce
is best used for aggregation operations , such as:
- Accumulate, multiply
- Merge strings or lists
- Multi-condition judgment combination
- Build nested structures (such as multi-layer dictionaries)
For example, if you want to calculate the factorial, you can write it like this:
from functools import reduce factorial = reduce(lambda x, y: x * y, range(1, 6)) # 1*2*3*4*5 print(factorial) # output 120
Or you have a set of strings that you want to spell into a complete sentence:
words = ['Hello', 'world', 'in', 'Python'] sentence = reduce(lambda x, y: x ' ' y, words) print(sentence) # output "Hello world in Python"
In this case, using reduce
is more compact than writing loops.
Some common pitfalls and precautions
- A function must accept two parameters : because
reduce
takes two values for operation each time, the function passed to it must be able to process two inputs. - Be careful with empty lists : If the passed list is empty and
initializer
is not set, an error will be reported. - Performance issues : Although
reduce
is written concisely, if the logic is too complex, it may affect readability and may even be difficult to debug. - When you can use alternatives, don't force reduce : for example, you can use
sum()
directly to sum, and you can use''.join()
to connect strings. These are more intuitive thanreduce
.
To give a counterexample, although the following code can run, it doesn't look clear enough:
reduce(lambda acc, x: acc.update({x: x**2}) or acc, [1,2,3], {})
The purpose of this line of code is to generate a dictionary, where key and value are squared relationships. But in order to implement this function, use .update()
and add or acc
to return the value. In this case, using a normal for loop is more clear.
Summarize the usage tips
- Use
reduce
as a "gradual merger" tool. - Try to avoid complex logic unless you can ensure that others can easily understand it.
- The initial value is a good thing, especially when you are not sure about inputting data.
Basically that's it. After mastering it, you will find that in some scenarios it can indeed make the code much more refreshing.
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