ENH: Series Mapping `na_action=ignore` result is misleading.

[chapmanderek]

Is your feature request related to a problem?

More a misleading flag... When mapping a dictionary to a pandas series and using the na_action='ignore' i would expect it to ignore unknown values. Currently it replaces them with an NaN. For example:

pd.Series(['calf', 'foal', 'bunny']).map({'calf':'cow','bunny':'rabbit'}, na_action='ignore')

returns:

0       cow
1       NaN
2    rabbit

Describe the solution you'd like

I would expect it to ignore items that aren't in the dictionary instead of replacing them. I would expect this to be returned:

0       cow
1       foal
2    rabbit

The current behavior is problematic for large dataframes with many different elements in a series where perhaps you only want to rename some of them. In this case you would have to either do a replace for each one... or make a mapping for every one and hope you didn't miss one.

Describe alternatives you've considered

Trying to differentiate the current action of ignore and the action of ignoring-but-not-replacing will be difficult. Perhaps a new value for na_action of dont_replace so that it is very apparent what you are asking to happen with NaNs while keeping the current behavior (albeit confusing) of ignore.

Additional context

Similar-ish to #14210

[rhshadrach]

With ser being the pandas Series:

ser = ser.map(mapper).fillna(ser)

will map only values that exist in mapper, leaving other values untouched.

i would expect it to ignore unknown values

I could see that as a reasonable interpretation when viewing the name alone, but do you have that same expectation from the documentation?

[chapmanderek]

@rhshadrach
No i think the documentation spells it out. It specifically says:

If ‘ignore’, propagate NaN values

Also one of the examples ("rabbit" is missing) it fills in with a NaN value. I just think that the actual flag is misleading. The label isnt the contents of the can.

The ignore flag only really makes sense in terms of the last example where the value is getting passed onto another function or thing. When you are trying to replace items (possibly the more common scenario) it doesn't work as I would expect given the name.

[topper-123]

For what it's worth, I think mappings using the existing value would be more logical to me than replacing it with Nan. But I agree the current behavior is as intended.

[topper-123]

Thinking a bit further, I think it's probably always better to use the Series.replace method for this:

>>> import pandas as pd
>>> ser = pd.Series(['calf', 'foal', 'bunny'])
>>> ser.map({'calf':'cow','bunny':'rabbit'}, na_action='ignore')
0       cow
1       NaN
2    rabbit
dtype: object
>>> ser.replace({'calf':'cow','bunny':'rabbit'})
0       cow
1      foal
2    rabbit
dtype: object

IMO we should add replace under the See also doc section under Series.map.

Thinking even further, should we not just deprecate allowing dict-likes to Series.map and direct users to use Series.replace for dict-likes instead? That seems very logical to me to do, especially if we see Series.map as a element-level version of Series.apply after #52140.

@rhshadrach

[rhshadrach]

It seems natural for Series.map to take a mapping. While there is an overlap in functionality here, I lean toward not deprecating unless there is an issue with map taking a mapping.