You can then iterate over all elements in the iterator, one element at a time, in a for or while loop such as:
for x in iter([1, 2, 3]).
Before we learn more about the
__iter__() dunder method, let’s have a look at a basic
customers = ['Alice', 'Bob', 'Carl', 'Dave', 'Elena', 'Frank'] iterator = iter(customers) print(next(iterator)) print(next(iterator)) for x in iterator: print(x)
You can view an explanation and the output of this on our detailed blog tutorial here:
Example Custom __iter__()
class Data: def __init__(self, data): self.data = data # an iterable def __iter__(self): self.current_index = 0 return self def __next__(self): if self.current_index < len(self.data): x = self.data[self.current_index] self.current_index += 1 return x raise StopIteration
__init__()initializes the data attribute that is expected to be an iterable.
__iter__()returns the iterator object — the one that implements the
__next__()method. In our case, this is the Data object on which it is called itself. We initialize
current_indexwith zero, so we start iterating with the first index of
__next__()returns the next value after one iteration. We increment the
current_indexattribute to keep track of the current index of the element in
Let’s create a
d and an iterator over the data object using the built-in
iter() function (that internally calls
__iter__())—and start iterating over the object using the built-in
next() function (that internally calls
d = Data([1, 'Alice', 42, 'finxter']) # Create an iterator iterator = iter(d) # Dynamically generate the next values - iterate! print(next(iterator)) print(next(iterator)) print(next(iterator)) print(next(iterator)) print(next(iterator))
The output is as follows: The first four calls result in the expected elements of the data attribute, i.e.,
'finxter'. The fifth call of
next() results in a
StopIteration error because we have finished iterating over all elements.
1 Alice 42 finxter Traceback (most recent call last): File "C:\Users\xcent\Desktop\code.py", line 34, in <module> print(next(iterator)) File "C:\Users\xcent\Desktop\code.py", line 14, in __next__ raise StopIteration StopIteration
TypeError: ‘…’ object is not iterable
If you call
iter(x) on an object on which the
x.__iter__() dunder method is not defined, Python will raise a
TypeError: '...' object is not iterable.
To fix this error, simply define the
__iter__() method in the class definition before calling
iter() on an object—and make sure that
__iter__() returns an iterator object on which the dunder method
__next__() is defined!
Here’s an example:
class Data: def __init__(self, data): self.data = data # an iterable d = Data([1, 'Alice', 42, 'finxter']) # Create an iterator iterator = iter(d)
Here’s the error message:
Traceback (most recent call last): File "C:\Users\xcent\Desktop\code.py", line 10, in <module> iterator = iter(d) TypeError: 'Data' object is not iterable
Where to Go From Here?
Enough theory. Let’s get some practice!
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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
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