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pythonbuilt-in

About the built-in __iter__

I'm learning python and reading fluent python book! While following some class implementation, I stopped by this snippet of code:

def __iter__(self):
    return iter(self._components)

with components being an array of floats, my question is: why calling the iter() method on components although it's already an iterable?.

Solution

  • While the documentation doesn't make it very clear, it is because __iter__ must (not should) return an iterator, not an iterable:

    % python
Python 3.6.3 (default, Oct  3 2017, 21:45:48) 
[GCC 7.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> class Foo:
...     def __iter__(self):
...         return []
... 
>>> iter(Foo())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: iter() returned non-iterator of type 'list'