ENH: usecols now accepts an int when only one column has to be read by I--P · Pull Request #6656 · numpy/numpy · GitHub
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ENH: usecols now accepts an int when only one column has to be read #6656

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Merged
seberg merged 1 commit into from
Feb 1, 2016
Merged

ENH: usecols now accepts an int when only one column has to be read #6656

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5 changes: 5 additions & 0 deletions doc/release/1.12.0-notes.rst
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35 changes: 30 additions & 5 deletions numpy/lib/npyio.py
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Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,7 @@
import itertools
import warnings
import weakref
from operator import itemgetter
from operator import itemgetter, index as opindex

import numpy as np
from . import format
Expand Down Expand Up @@ -714,10 +714,18 @@ def loadtxt(fname, dtype=float, comments='#', delimiter=None,
``converters = {3: lambda s: float(s.strip() or 0)}``. Default: None.
skiprows : int, optional
Skip the first `skiprows` lines; default: 0.
usecols : sequence, optional
Which columns to read, with 0 being the first. For example,
``usecols = (1,4,5)`` will extract the 2nd, 5th and 6th columns.

usecols : int or sequence, optional
Which columns to read, with 0 being the first. For example,
usecols = (1,4,5) will extract the 2nd, 5th and 6th columns.
The default, None, results in all columns being read.

.. versionadded:: 1.11.0

Also when a single column has to be read it is possible to use
an integer instead of a tuple. E.g ``usecols = 3`` reads the
third column the same way as `usecols = (3,)`` would.

unpack : bool, optional
If True, the returned array is transposed, so that arguments may be
unpacked using ``x, y, z = loadtxt(...)``. When used with a structured
Expand Down Expand Up @@ -786,8 +794,25 @@ def loadtxt(fname, dtype=float, comments='#', delimiter=None,
user_converters = converters
if delimiter is not None:
delimiter = asbytes(delimiter)

if usecols is not None:
usecols = list(usecols)
# Allow usecols to be a single int or a sequence of ints
try:
usecols_as_list = list(usecols)
except TypeError:
usecols_as_list = [usecols]
for col_idx in usecols_as_list:
try:
opindex(col_idx)
except TypeError as e:
e.args = (
"usecols must be an int or a sequence of ints but "
"it contains at least one element of type %s" %
type(col_idx),
)
raise
# Fall back to existing code
usecols = usecols_as_list

@seberg seberg Jan 4, 2016

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Actually, I think there is no reason to create usecols_as_list at all. Also the comment is rather unnecessary "existing code" is not existing code anymore as soon as we put this in ;).

@I--P I--P Jan 4, 2016

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By existing code I meant all the lines below this one where usecols is used.
You're right I can use usecols and avoid the creation of usecols_as_list. I wished to make this easier to read, I hate code like foo=some_fancy_type(foo) buried deep in some code and then waste time trying to understand why foo is not any more what it was at the beginning.


fown = False
try:
Expand Down
38 changes: 38 additions & 0 deletions numpy/lib/tests/test_io.py
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