CODEWITHSUNDEEP
pythonfor-loop
Hackxx
Hackxx

Reputation: 71

Multi variable for loop python

I know that you can do nested for loops, for example:

for ch in a:
    for ch2 in ch:
        print(ch2)

However, I've seen for loops that go like this:

for ch, ch2 in a:
    # blah blah

Are these two loops equivalent? Or does the second loop do something different than the first?

Upvotes: 1

Views: 306

Answers (1)

Two-Bit Alchemist
Two-Bit Alchemist

Reputation: 18457

No, they are not.

The second is an example of multiple assignment. If you assign to a tuple, Python unpacks the values of an iterable into the names you gave.

The second loop is rather equivalent to this:

for seq in a:
    ch = seq[0]
    ch2 = seq[1]
    # blah blah

As @Kasramvd points out in a comment to your question, this only works if a is a sequence with the correct number of items. Otherwise, Python will raise a ValueError.

Edit to address dict iteration (as brought up in comment):

When you iterate over a Python dict using the normal for x in y syntax, x is the key relevant to each iteration.

for x in y:   # y is a dict
    y[x]      # this retrieves the value because x has the key

The type of loop you are talking about is achieved as follows:

for key, val in y.items():
    print(key, 'is the key')
    print('y[key] is', val)

This is still the same kind of unpacking as described above, because dict.items gives you a list of tuples corresponding to the dict contents. That is:

d = {'a': 1, 'b': 2}
print(d.items())    # [('a', 1), ('b', 2)]

Upvotes: 5

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