Itertools.combinations is used to generate all non-repetitive combinations (order irrelevant) that selects a specified number of elements from the iterable object. Its usage includes: 1. Select 2 element combinations from the list, such as ('A', 'B'), ('A', 'C'), etc., to avoid duplicate order; 2. Take 3 character combinations of strings, such as "abc" and "abd", which are suitable for subsequence generation; 3. Find the combinations whose sum of two numbers is equal to the target value, such as 1 5=6, simplify the double loop logic; the difference between combinations and arrangement lies in whether the order is important. Combinations regard AB and BA as the same, while permutations are regarded as different; this tool is suitable for course selection, team formation, game competition, feature combination and algorithm enumeration, and the code is concise and efficient, suitable for dealing with the problem of "selecting but not ranking".
itertools.combinations
is a very practical tool in Python to generate all non-repetitive combinations (independent order) that select a specified number of elements from a given iterable object. The following are some common examples to illustrate its usage.

What are combinations
?
itertools.combinations(iterable, r)
-
iterable
: input data, such as lists, strings, etc. -
r
: The number of elements selected each time - Returns an iterator containing all combinations of length
r
, non-repeat and out of order
Example 1: Select a combination of 2 elements from the list
import itertools items = ['A', 'B', 'C', 'D'] comb = itertools.combinations(items, 2) for c in comb: print(c)
Output:

('A', 'B') ('A', 'C') ('A', 'D') ('B', 'C') ('B', 'D') ('C', 'D')
Note:
('A', 'B')
and('B', 'A')
are considered the same combination, so they only appear once.
Example 2: Take a combination of 3 characters in a string
import itertools s = "abcd" comb = itertools.combinations(s, 3) for c in comb: print(''.join(c))
Output:

abc abd acd bcd
This is useful in generating codons, subsequences and other scenarios.
Example 3: Find the combination of the sum of two numbers in the numeric list equals the target value
import itertools numbers = [1, 2, 3, 4, 5] target = 6 comb = itertools.combinations(numbers, 2) for a, b in comb: if ab == target: print(f"{a} = {target}")
Output:
1 5 = 6 2 4 = 6
Avoid manual writing double loops, making the code clearer.
Tips: Combination vs Arrangement
-
combinations
: order is irrelevant,AB
andBA
are one -
permutations
: The order is related,AB
andBA
are two types
list(itertools.combinations('AB', 2)) # [('A', 'B')] list(itertools.permutations('AB', 2)) # [('A', 'B'), ('B', 'A')]
Practical application scenarios
- Select a combination of several items from multiple options (such as: selecting courses, forming teams)
- Enumerate all possible pairings (such as match vs.)
- Attempts to combine features in data analysis
- Violent enumeration solution space in algorithm problems
Basically that's it. itertools.combinations
is simple to use, but it is especially suitable for dealing with the problem of "selecting but not ranking". It is much cleaner to write than nested for loops.
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