The Python 'zip' function

The Python 'zip' function returns a list of tuples, where the i-th tuple contains the i-th element from the each of the argument sequences. I learnt about 'zip' in the Python course in Cambridge, and my colleague Bhavana Harsha recently reminded me about it.

Here are some examples of using 'zip':

letters = ['A', 'B', 'C', 'D', 'E']
numbers = [1, 2, 3, 4, 5]
for x, y in zip(letters, numbers):
    print(x, y)
A 1
B 2
C 3
D 4
E 5

mystring = 'gene1 species1 gene2 species2 gene3 species3 gene4 species4'
temp = mystring.split()
pairs = zip(temp[0::2], temp[1::2])
for x, y in pairs:
    print(x, y)
gene1 species1
gene2 species2
gene3 species3
gene4 species4
Note: here temp[0::2] gives the 1st, 3rd, 5th... element in the list 'temp', and temp[1::2] gives the 2nd, 4th, 6th... element in the list 'temp'.

seq1 = 'ACGTAT'
seq2 = 'ACTTTT'
bases = zip(seq1,seq2)
for x, y in bases:
    print(x, y)
A A
C C
G T
T T
A T
T T

Note that once you have expanded a 'zip' once (eg. printing it out), then it no longer holds the data. For example, if you try and print out the zip 'bases' in the last example a second time, then you won't get any result.

Thanks to Bhavana Harsha for the last example.