BUG/ENH: provide better .loc based semantics for float based indicies, continuing not to fallback (related GH236) by jreback · Pull Request #4850 · pandas-dev/pandas (original) (raw)
Since there's obviously no label-based indexing going on here, we only have
to consider what should happen for integer/offset-based indexing.
The general python rule for integer/offset-based indexing (used by e.g.
lists), and the one that numpy will switch to in 1.9 or so, is that using
an object with type 'float' is always an error, no matter whether this
float happens to have an integer value. (Or more specifically, the rule is
that an object must either be of type 'int', or else it must have a
.index() method which returns an int.) So every example there with
[2.0] should become an error eventually, possibly after a deprecation
period.
The one other questionable case for me is s.loc[2]. I actually think this
should be an error -- currently .loc's logic is quite confusing and
.ix-like, where when it sees an integer it uses some heuristics to guess
whether this is a label or an offset. AFAICT the rule right now is:
- if you have an Int64Index, then that means that all labels are integers,
and THEREFORE all integers passed to loc must be labels. - if you have a generic Index, then your labels could be any random
(hashable) python objects; and in particular, they might or might not be
integers, and THEREFORE integers passed to loc might or might not be labels.
The problem is that when you actually lay out the logic like this you can
see that those two THEREFOREs are completely invalid conclusions, logically
speaking :-). Straight up post hoc ergo propter hoc.
IMHO to make .loc predictable, the fallback to integer/offset-based
indexing should only be allowed if the Index can guarantee that no labels
are integers. (And by "integers" we mean, compare == to a Python integer.
Which Python floats can do.) So that would mean that .loc ought to be able
to do integer/offset-based indexing for DatetimeIndex and MultiIndex, but
not Index or Int64Index or Float64Index.
In practice I guess the only common situations where people use
plain-old-generic 'Index' right now, once Float64Index is added, is for
all-string indexes. If the above rule is adopted -- the one that says for a
generic Index that can contain anything, .loc must always be label based,
never fall back to being integer/offset-based -- then people using
all-string indexes would probably be happier if a specialized StringIndex
was created that relaxed this rule.
On Wed, Sep 18, 2013 at 8:48 PM, jreback notifications@github.com wrote:
s = Series([0,1,2],['foo','bar'','bar'])
what should this yield for (this is why this is tricky) as you can make a
case that s[2.0] shouln't work
at all, but at the same time when do you decide thats[2]
s[2.0]
s.loc[2]
s.loc[2.0]
s.ix[2]
s.ix[2.0]
s.iloc[2]
s.iloc[2.0]
``—
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