Question

I’m trying to use ‘izip’ to read lines simultaneously from two files, however got errors like below:

>>> f1=open('/home/xug/scratch/test/test_1.fastq','r')
>>> f2=open('/home/xug/scratch/test/test_2.fastq','r')
>>> from itertools import izip
>>> for i,line1,line2 in izip(f1,f2):
...     if i%4==3:
...        print line1,line2
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: need more than 2 values to unpack

I don’t know why….what is “need more than 2 values”?
thx

Answer 1

izip() simply zips the two iterables to pairs. It doesn’t introduce an additional counter, as you seem to expect. Try using enumerate()

for i, (line1, line2) in enumerate(izip(f1, f2)):
    ...

to also get that counter.

The error message results from the attempt to assign the first item of your iterable to i, line1, line2. Since the first item is a pair of strings, say s1 and s2, this assignment would be essentially equivalent to

i, line1, line2 = s1, s2

which would clearly needs more than two values to unpack. (To be precise, it would need three.)