The voodoo of zip(*someList)

[Message Drop Box]

All,

How (and why) does zip(*someList) work?

s = [[1, 2, 3], ['one', 'two', 'three'], ['I', 'II', 'III']]
zip(*s)

[(1, 'one', 'I'), (2, 'two', 'II'), (3, 'three', 'III')]

I've never seen the star '*' outside of function/method definitions
and I've looked in the Python documentation without success. This
syntax is voodoo to me at the moment. I'm stumped.

Thanks,
Stuart

[David Wilson]

Message Drop Box wrote:

All,

How (and why) does zip(*someList) work?

s = [[1, 2, 3], ['one', 'two', 'three'], ['I', 'II', 'III']]
zip(*s)

[(1, 'one', 'I'), (2, 'two', 'II'), (3, 'three', 'III')]

I've never seen the star '*' outside of function/method definitions
and I've looked in the Python documentation without success. This
syntax is voodoo to me at the moment. I'm stumped.

Thanks,
Stuart

Help on built-in function zip:

zip(...)
zip(seq1 [, seq2 [...]]) -> [(seq1[0], seq2[0] ...), (...)]

Return a list of tuples, where each tuple contains the i-th element
from each of the argument sequences. The returned list is truncated
in length to the length of the shortest argument sequence.
On the asterisk syntax:
http://docs.python.org/tut/node6.htm...00000000000000
http://docs.python.org/ref/calls.html
Basically,

your_sequence = [ 1, 2, 3, 4 ]
zip(*your_sequence)

is equivalent to:

zip(1, 2, 3, 4)

In other words, it allows you to programmatically build an argument
list. Similarly,

your_mapping = { 'cmpfunc': lambda x: x }
some_list = [ 3, 1, 6, 2 ]
some_list.sort(**your_mapping)

is equivalent to:

zip(cmpfunc = lambda x: x)

The zip function takes an item from each of the sequences given as
arguments, and packs them into a tuple, continuing until the sequences
are exhausted.

seq1 = [ 1, 2, 3, 4 ]
seq2 = [ 4, 3, 2, 1 ]
seq3 = "dave"

zip(seq1, seq2) is:
[ (1, 4), (2, 3), (3, 2), (4, 1) ]

zip(seq1, seq2, seq3) is:
[ (1, 4, "d"), (2, 3, "a"), (3, 2, "v"), (4, 1, "e") ]

There is also an itertools.izip which does the same thing in a
generator. HTH,
David.

[David Wilson]

David Wilson wrote:

In other words, it allows you to programmatically build an argument
list. Similarly,

your_mapping = { 'cmpfunc': lambda x: x }
some_list = [ 3, 1, 6, 2 ]
some_list.sort(**your_mapping)

is equivalent to:

zip(cmpfunc = lambda x: x)

some_list = [ 3, 1, 6, 2 ]
some_list.sort(cmpfunc = lambda x: x)

There's nothing more confusing when you are trying to learn than an
incorrect example. My apologies, lacking sleep here. :)
David.

[Jacek Generowicz]

md****@yahoo.com (Message Drop Box) writes:

How (and why) does zip(*someList) work?
[...]
I've never seen the star '*' outside of function/method definitions
It can also be used in function invocations, where it means pretty
much the reverse of what it means in function definitions.

In a parameter list "*foo" means "collect remaining positional
arguments into a sequence (tuple) called 'foo'". In an argument list,
it means "take the sequence 'foo' and expand it into a set of
positional arguments".

You can "reverse" the **kwds syntax in the same way too.
and I've looked in the Python documentation without success.

Try page 39 of the Python Reference Manual.

[Message Drop Box]

Thank you all very much.

Jacek Generowicz <ja**************@cern.ch> wrote

I've never seen the star '*' outside of function/method definitions

It can also be used in function invocations, where it means pretty
much the reverse of what it means in function definitions.

In a parameter list "*foo" means "collect remaining positional
arguments into a sequence (tuple) called 'foo'". In an argument list,
it means "take the sequence 'foo' and expand it into a set of
positional arguments".

You can "reverse" the **kwds syntax in the same way too.

and I've looked in the Python documentation without success.

Try page 39 of the Python Reference Manual.