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Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →In Python, for item in container asks the object for an iterator, while item in container asks whether the object contains that item. A custom container can define __iter__() to control what iteration yields and __contains__() to define membership directly. If it has no __contains__(), Python falls back to iteration and then to the legacy indexed-sequence protocol.
What is the difference between __iter__ and __contains__?
__iter__() supports iteration: it returns an iterator whose values are requested one at a time. __contains__(item) supports the in and not in operators by deciding whether the item belongs to the object. These hooks serve related but distinct questions: “What can I get from this object?” and “Does this object contain this item?”
An iterable container and its iterator are related roles, but they do not have to be the same object. A container’s __iter__() should return an iterator. That iterator implements __next__() to provide successive values and __iter__() to return itself. See the Python built-in types reference for the iterator protocol.
How does Python check if an item is in an object?
Python first uses the object’s __contains__() method when it has one. If the method is absent, Python tries iteration with __iter__(); if that is unavailable, it tries the older sequence protocol through __getitem__(). The Python Language Reference documents this fallback order.
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During an iteration-based membership check, Python compares each yielded value with the sought item using identity-or-equality matching. In practical terms, a match succeeds if an item is the same object or compares equal to it. String and bytes membership are different: they test whether the left operand occurs as a substring of the right operand. The membership-test reference describes these rules.
Legacy indexed iteration
For the __getitem__() fallback, Python requests items at nonnegative indexes in order, starting at zero. The object signals that there are no more items by raising IndexError. Other exceptions are not end-of-sequence signals and propagate as errors. This is a compatibility protocol; new container classes should generally provide __iter__() rather than rely on indexed fallback.
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What should membership mean for a mapping or sequence?
For a mapping, the conventional meaning of iteration and membership is keys. Thus, "name" in mapping checks whether "name" is a key; it does not search the mapping’s values or key-value pairs. For a sequence, iteration yields values and membership searches those values. A custom type should keep these operations consistent with the kind of container it represents.
How to implement both methods in a custom container
This reusable container stores a set of allowed labels. Iteration exposes those labels, and membership checks the same set directly:
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class LabelSet:
def __init__(self, labels):
self._labels = set(labels)
def __iter__(self):
return iter(self._labels)
def __contains__(self, item):
return item in self._labels
labels = LabelSet(["draft", "review", "published"])
for label in labels:
print(label)
print("review" in labels) # TrueHere, __iter__() returns an iterator over the intended items, while __contains__() delegates the membership decision to the backing set. A direct membership method can avoid walking every yielded item when the backing structure supports a suitable lookup, or can express a membership rule that does not depend on iteration. The actual performance depends on the data structure and implementation; the method name alone does not guarantee a particular speed.
Choose the behavior deliberately
- Define what counts as a member. Decide whether membership means a key, a stored value, a substring, or a domain-specific condition.
- Choose what iteration yields. Ensure
__iter__()exposes the items callers expect. For a reusable container, each call should normally provide a fresh iterator. - Use direct lookup when it matches the design. Implement
__contains__()against an appropriate index or backing structure if that gives the correct membership semantics. - Prefer the modern iteration protocol. Implement
__iter__()for a new iterable instead of depending only on__getitem__()fallback.
Can an object support membership without iteration?
Yes. A class can implement __contains__() without implementing __iter__(). Such an object can answer item in object but cannot necessarily be used in a for loop. This is useful when the membership question is meaningful but exposing a sequence of items is not part of the object’s interface.
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The reverse distinction matters for one-shot iterators. If an iterator does not define __contains__(), membership can test it by iteration. Checking for an item may advance the iterator through that item; checking for an absent item may consume it to exhaustion. This does not apply to every iterable: a reusable container can return a fresh iterator on each call, while a generator or other one-shot iterator represents ongoing state.
Quick Recap
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