DOC: Series.update throws a `FutureWarning` about `def[col] = df[col].method` but `.update` returns `None` and works `inplace`

[spawn-guy]

Pandas version checks

• [X] I have checked that the issue still exists on the latest versions of the docs on main here

Location of the documentation

https://pandas.pydata.org/docs/dev/reference/api/pandas.Series.update.html#pandas.Series.update

Documentation problem

df.update resembles how python.dict.update works, but df.update doesn't support CoW

Suggested fix for documentation

remove FutureWarning for the df.update

or create a (for example) df.coalesce method that will, actually, return something. this shouldn't brake existing code

[rhshadrach]

df.update doesn't support CoW

Thanks for the report - can you provide a reproducible example on how CoW is not supported.

[spawn-guy]

@rhshadrach here is some code and log

# select best source: heading
# HeadingTrue > HeadingMagnetic > HeadingAndDeclination (this is also magnetic) > TrackMadeGood
measurements_df["heading"] = measurements_df["gps_course_over_ground"]
# replace if other value is not nan
measurements_df["heading"].update(measurements_df["gps_heading"])

FutureWarning

_task.py:427: FutureWarning: A value is trying to be set on a copy of a DataFrame or Series through chained assignment using an inplace method.
The behavior will change in pandas 3.0. This inplace method will never work because the intermediate object on which we are setting values always behaves as a copy.

For example, when doing 'df[col].method(value, inplace=True)', try using 'df.method({col: value}, inplace=True)' or df[col] = df[col].method(value) instead, to perform the operation inplace on the original object.

  measurements_df["heading"].update(measurements_df["gps_heading"])

Inconsistency with the warning:

• update is always "inplace=True"
• there is no df[col] = df[col].update(value) that actually "returns" something

[rhshadrach]

Thanks @spawn-guy, however your example is not reproducible because you did not provide measurements_df. Can you provide a reproducible example?