In datatstructures.py, there is a method values():
def values(self):
"""Iterate over all values."""
for item in self:
yield item[0]
self is an instance of the class; how can it be iterated over?
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Possible duplicate of Python iterating through object attributessobolevn– sobolevn2016-01-24 17:58:47 +00:00Commented Jan 24, 2016 at 17:58
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Not a duplicate. My question is not How to but rather How can (as in: how can it be possible).Pyderman– Pyderman2016-01-24 18:03:19 +00:00Commented Jan 24, 2016 at 18:03
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4 Answers
The class which the given method belongs to extends ImmutableList which is iterable, thus the method can iterate on the class.
1 Comment
Pyderman
What defines
ImmutableList as iterable? I see now __iter__ method.Simple, it has to implement __iter__ method, e.g.
class Test:
def __iter__(self):
yield 1
yield 2
>>> instance = Test()
>>> for val in instance:
... print val
...
1
2
Comments
My question is not How to but rather How can (as in: how can it be possible)
self refers to the actual tangible object you deal with, classes are more like their interface (don't interpret this too strictly); if an instances' class defines a __iter__ (or __getitem__) method they can be iterated over in a for loop. PEP 234 deals with the semantics and implementation of iterators.
In your specific case the function is a generator which has nothing to do with the __iter__ method, it simply transforms the values function to a generator in order to support iteration of the form:
for i in instance.values(): # do stuff
If the object doesn't define an __iter__ it won't be able to be iterated over, as an example:
class myfoo:
def func(self):
for i in range(10): yield i
f = myfoo()
The instance f is now not iterable:
for i in f: print(i) # TypeError
On the other hand, we can use func in a for loop:
for i in f.func(): print(i, end=" ")
0 1 2 3 4 5 6 7 8 9
Changing func to __iter__ changes the picture, now the instance f is iterable:
class myfoo:
def __iter__(self):
for i in range(10): yield i
f = myfoo()
Iterating through f is done intuitively with:
for i in f: print(i, end=" ")
0 1 2 3 4 5 6 7 8 9
Asking why, as in, how can this be possible is like asking why can strings be multiplied with "s" * 4. This is the way things were implemented because it just made sense as a way of handling the common case of looping through the contents of a container. Don't overthink things, it isn't necessary.
answered Jan 24, 2016 at 18:23
Dimitris Fasarakis Hilliard
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1 Comment
Dimitris Fasarakis Hilliard
@Pyderman You're welcome, good for you for digging in the source code of projects and trying to understand how they work.