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?
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)]
dict iteration and how the above notion fits into it.for x in y.items(), x would be a tuple each time, but since you provide two assignment targets, Python realizes and unpacks it for you so you have the individual elements.
aonly works ifahas duplex items.