[Python] np.loadtxt skips unexpected lines when reading strings

I have a file structured as such:

IID SuperPops
FGXXXXXXXX R6
FGXXXXXXXX R12


Containing 524123 lines.

I need to read the first column as an array:

len(np.loadtxt('foo.txt',usecols=0,dtype=str))
524123


But if i also try to skip the header all of a sudden 10 lines go missing:

len(np.loadtxt('foo.txt',usecols=0,dtype=str,skiprows=1))
524112


This does not happen if i change type though:

len(np.loadtxt('foo.txt',usecols=0,dtype='<U20',skiprows=1))
524122
len(np.loadtxt('foo.txt',usecols=0,dtype='<U10',skiprows=1))
524122


The 10 missing entries are all of length 10:

[len(elem) for elem in set(np.loadtxt('foo.txt',usecols=0,dtype='<U20',skiprows=1)) - set(np.loadtxt('foo.txt',usecols=0,dtype=str,skiprows=1))]
[10, 10, 10, 10, 10, 10, 10, 10, 10, 10]


What could be the reason and why is it specifically triggered by skiprows?

Continue reading...