Python基础知识：对于我来说，枚举（seq）中的元素..为什么/如何工作？

Question

Hello I've found an intersting snippet:

seq = ["one", "two", "three"] #edited
for i, element in enumerate(seq):
    seq[i] = '%d: %s' % (i, seq[i])

>>> seq
['0: one', '1: two', '2: three']

I wonder how python is doing that.... for me element should be undefined...but obviously it isn't..what does python do here?

Thanks a lot!

Answer 1

enumerate essentially turns each element of the input list into a list of tuples with the first element as the index and the element as the second. enumerate(['one', 'two', 'three']) therefore turns into [(0, 'one'), (1, 'two'), (2, 'three')]

The bit just after the for pretty much assigns i to the first element and element to the second for each tuple in the enumeration. So for example in the first iteration, i == 0 and element == 'one' , and you just go through the other tuples to get the values for the other iterations.

Answer 2

The actual definition of enumerate function is like this

enumerate(iterable[, start]) -> iterator for index, value of iterable

Return an enumerate object.  iterable must be another object that supports
iteration.  The enumerate object yields pairs containing a count (from
start, which defaults to zero) and a value yielded by the iterable argument.
enumerate is useful for obtaining an indexed list:
    (0, seq[0]), (1, seq[1]), (2, seq[2]), ...

But the parameter seq if not defined will not return the desired output, rather it will throw an error like- NameError: name 'seq' is not defined.

Answer 3

i am not sure what you want, to know enumerate or what's happening here in this . may this can help :

seq = ["one", "two", "three"]
for i range(3):
    seq[i] = '%d: %s' % (i, seq[i])

>>> seq
 ['0: one', '1: two', '2: three']

actually , here "seq" is object of list-class. Each element in list reference to independent object of different class . when in above function ; first you get the reference of each list element's object by accessing "seq[i]", you create a new object using referenced object value (can belongs to other class), now u again referencing the "seq[i]" to new created object for each element .

Answer 4

You are right. seq is not defined. Your snippet will not work. It will only work if seq has ben set to seq = ['one', 'two', 'three'] before. Just try it!