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DOC: read_csv usecols element order is ignored #18729

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Dec 21, 2017
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DOC: read_csv usecols element order is ignored #18729

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5 changes: 3 additions & 2 deletions doc/source/io.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -131,7 +131,8 @@ usecols : array-like or callable, default ``None``
be positional (i.e. integer indices into the document columns) or strings
that correspond to column names provided either by the user in `names` or
inferred from the document header row(s). For example, a valid array-like
`usecols` parameter would be [0, 1, 2] or ['foo', 'bar', 'baz'].
`usecols` parameter would be [0, 1, 2] or ['foo', 'bar', 'baz']. Element
order is ignored, so usecols=[0,1] is the same as [1, 0].

If callable, the callable function will be evaluated against the column names,
returning names where the callable function evaluates to True:
Expand Down Expand Up @@ -2822,11 +2823,11 @@ to be parsed.

If `usecols` is a list of integers, then it is assumed to be the file column
indices to be parsed.

.. code-block:: python

read_excel('path_to_file.xls', 'Sheet1', usecols=[0, 2, 3])

Element order is ignored, so usecols=[0,1] is the same as [1,0].

Parsing Dates
+++++++++++++
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3 changes: 2 additions & 1 deletion pandas/io/parsers.py
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Expand Up @@ -101,7 +101,8 @@
be positional (i.e. integer indices into the document columns) or strings
that correspond to column names provided either by the user in `names` or
inferred from the document header row(s). For example, a valid array-like
`usecols` parameter would be [0, 1, 2] or ['foo', 'bar', 'baz'].
`usecols` parameter would be [0, 1, 2] or ['foo', 'bar', 'baz']. Element
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can you also add this note to io.rst

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Done.

order is ignored, so usecols=[1,0] is the same as [0,1].

If callable, the callable function will be evaluated against the column
names, returning names where the callable function evaluates to True. An
Expand Down