ShardedCorpus skips the first value of a generator

[karkkainenk1]

Description

ShardedCorpus skips the first value of a generator. This is possibly caused by ShardedCorpus not using the fixed corpus that is returned from is_corpus method, but I haven't verified this yet.

Steps/Code/Corpus to Reproduce

from gensim.corpora.sharded_corpus import ShardedCorpus

def my_generator():
    yield [(0,1)]
    yield [(1,1)]
    yield [(2,1)]

corpus = ShardedCorpus("corpus", my_generator(), dim=3, overwrite=True)

print(len(corpus))
print(corpus[0])

Expected Results

Expected output:

3
[ 1.  0.  0.]

Actual Results

Actual output:

2
[ 0.  1.  0.]

I.e. The first item in the generator has been skipped and is missing from the resulting corpus

Versions

Darwin-16.7.0-x86_64-i386-64bit
Python 3.6.1 |Anaconda 4.4.0 (x86_64)| (default, May 11 2017, 13:04:09)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)]
NumPy 1.12.1
SciPy 0.19.0
gensim 2.3.0
FAST_VERSION 1

[piskvorky]

@karkkainenk1 thanks for reporting and for the minimal example!

Some destructive peeking, or incorrect use of is_corpus, would be my guess too. Can you look into it?

[karkkainenk1]

Sure thing, sent a pull request.