Why am I getting a StopIteration error when using next()?

Question

I am not able to clarify my self over the use of next() in python(3).

I have a data :

chr pos ms01e_PI    ms01e_PG_al ms02g_PI    ms02g_PG_al ms03g_PI    ms03g_PG_al ms04h_PI    ms04h_PG_al
2   15881989    4   C|C 6   A|C 7   C|C 7   C|C
2   15882091    4   A|T 6   A|T 7   T|A 7   A|A
2   15882148    4   T|T 6   T|T 7   T|T 7   T|G

and I read it like:

Works fine

c = csv.DictReader(io.StringIO(data), dialect=csv.excel_tab)
print(c)
print(list(c))

Works fine

c = csv.DictReader(io.StringIO(data), dialect=csv.excel_tab)
print(c)
keys = next(c)
print('keys:', keys)

But, now there is a problem.

c = csv.DictReader(io.StringIO(data), dialect=csv.excel_tab)
print(c)
print(list(c))
keys = next(c)
print('keys:', keys)

Error message:

Traceback (most recent call last):
2   15882601    4   C|C 9   C|C 6   C|C 5   T|C

  File "/home/everestial007/Test03.py", line 24, in <module>
keys = next(c)
  File "/home/everestial007/anaconda3/lib/python3.5/csv.py", line 110, in __next__

    row = next(self.reader)

StopIteration

Why does print(keys) after print(list(c)) gives StopIteration? I read the documentation but I am not clear on this particular example.

Answer 1

The error isn't with the print statement. It's with the keys = next(c) line. Consider a simpler example which reproduces your issue.

a = (i ** 2 for i in range(5))  

a           # `a` is a generator object
<generator object <genexpr> at 0x150e286c0>

list(a)     # calling `list` on `a` will exhaust the generator
[0, 1, 4, 9, 16]

next(a)     # calling `next` on an exhausted generator object raises `StopIteration`
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-2076-3f6e2eea332d> in <module>()
----> 1 next(a)

StopIteration: 

What happens is that c is an iterator object (very similar to the generator a above), and is meant to be iterated over once until it is exhausted. Calling list on this object will exhaust it, so that the elements can be collected into a list.

Once the object has been exhausted, it will not produce any more elements. At this point, the generator mechanism is designed to raise a StopIteration if you attempt to iterate over it even after it has been exhausted. Constructs such as for loops listen for this error, silently swallowing it, however, next returns the raw exception as soon as it has been raised.