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Use A - B or A.difference(B) to get the elements that are in A but not in B. Set difference is directional: reversing the operands can change the result.
Subtract one set from another
Python’s set difference keeps members of the left-hand set that are absent from the set on the right. For example:
A = {1, 2, 3, 4}
B = {3, 4, 5}
print(A - B) # {1, 2}
print(A.difference(B)) # {1, 2}The two expressions return the same difference here. The printed order is illustrative: sets are unordered, so Python does not promise that elements will appear in that order. The Python 3.14.8 built-in types documentation defines the operation as returning elements in the set that are not in the others.
Choose the operator or the method
| Form | Example | What it accepts |
|---|---|---|
| Subtraction operator | left - right |
Set operands |
difference() method |
left.difference(right) |
Any iterable as an argument |
Use - when both operands are sets and you want concise syntax. Choose .difference() when the items to exclude come in a list, tuple, or another iterable:
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left = {"red", "blue", "green"}
remaining = left.difference(["blue"])
# {'red', 'green'}left - ["blue"] is not the corresponding operator form; the operator requires set operands. You could convert the list first with set(["blue"]), but conversion discards duplicates and sequence order. Elements added to a set must be hashable, so a list containing mutable lists cannot be converted into a set of those lists.
Exclude items from multiple sets
Pass multiple iterables to difference(), or chain subtraction. Each operand after the first is another collection of values to exclude from the original left-hand set:
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available = {"A", "B", "C", "D"}
blocked = {"B"}
reserved = {"D"}
open_items = available.difference(blocked, reserved)
# {'A', 'C'}; display order is not guaranteedThe equivalent chained form is available - blocked - reserved. In either form, the starting set is the source; the later operands supply exclusions.
Return a new set or update the original
- and .difference() produce a result without changing the original set. To remove excluded items from a mutable set itself, use difference_update() or its operator form, -=:
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original = {1, 2, 3}
removed = {2}
result = original.difference(removed) # original is still {1, 2, 3}
original.difference_update(removed) # original is now {1, 3}These update operations mutate set objects. A frozenset supports ordinary difference but is immutable and has no in-place update operation.
Difference is not symmetric difference
A - B is one-sided: it includes elements in A that are not in B. Symmetric difference is two-sided: it includes elements found in either set, but not in both. Use A ^ B or A.symmetric_difference(B) when you want those non-overlapping members from both sets.
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Sort only when you need ordered output
A set is an unordered collection of distinct hashable objects. It does not preserve insertion order and cannot be indexed or sliced. If you need repeatable presentation and the values can be compared, sort the result explicitly:
print(sorted(A - B))This creates a sorted list for display; the set-difference result itself remains unordered. Converting a sequence to a set also cannot preserve its duplicate counts or original order. Both set and frozenset support difference. With mixed set and frozenset binary operations, Python returns the type of the first operand.
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