Repository: pandas-dev/pandas
File: pandas/core/frame.py (Python)

Code before:
        >>> df = pd.DataFrame({'A': [1, 2, 3],
        ...                    'B': [400, 500, 600]})
        >>> new_df = pd.DataFrame({'B': [4, np.nan, 6]})
        >>> df.update(new_df)
        >>> df
           A      B
        0  1    4.0
        1  2  500.0
        2  3    6.0
        """
        from pandas.core.computation import expressions

        # TODO: Support other joins
        if join != "left":  # pragma: no cover
            raise NotImplementedError("Only left join is supported")
        if errors not in ["ignore", "raise"]:
            raise ValueError("The parameter errors must be either 'ignore' or 'raise'")

        if not isinstance(other, DataFrame):
            other = DataFrame(other)

        # reindex rows, non-matching columns get skipped
        other = other.reindex(self.index)

        for col in self.columns:
            if col not in other.columns:
                continue

            this = self[col]._values
            that = other[col]._values

            if filter_func is not None:
                with np.errstate(all="ignore"):
                    mask = ~filter_func(this) | isna(that)
            else:
                if errors == "raise":
                    mask_this = notna(that)
                    mask_that = notna(this)
                    if any(mask_this & mask_that):
                        raise ValueError("Data overlaps.")

                if overwrite:
                    mask = isna(that)
                else:
                    mask = notna(this)

            # don't overwrite columns unnecessarily
            if mask.all():
                continue

            self.loc[:, col] = expressions.where(mask, this, that)

Review comment (suggestion):
Could probably do a union of `self.columns.union(other.columns)` first and iterate over that instead of constantly doing `in`

Code after:

>>> df = pd.DataFrame({'Animal': ['Falcon', 'Falcon',
...                               'Parrot', 'Parrot'],
...                    'Max Speed': [380., 370., 24., 26.]})
>>> df.groupby("Animal", group_keys=True).apply(lambda x: x)
          Animal  Max Speed
Animal
Falcon 0  Falcon      380.0
       1  Falcon      370.0
Parrot 2  Parrot       24.0
       3  Parrot       26.0

>>> df.groupby("Animal", group_keys=False).apply(lambda x: x)
   Animal  Max Speed
0  Falcon      380.0
1  Falcon      370.0
2  Parrot       24.0
3  Parrot       26.0
"""
    )
    @Appender(_shared_docs["groupby"] % _shared_doc_kwargs)
    def groupby(
        self,
        by=None,
        axis: Axis = 0,
        level: IndexLabel | None = None,
        as_index: bool = True,
        sort: bool = True,
        group_keys: bool | lib.NoDefault = no_default,
        observed: bool = False,
        dropna: bool = True,
    ) -> DataFrameGroupBy:
        from pandas.core.groupby.generic import DataFrameGroupBy

        if level is None and by is None:
            raise TypeError("You have to supply one of 'by' and 'level'")
        axis = self._get_axis_number(axis)

        return DataFrameGroupBy(
            obj=self,
            keys=by,
            axis=axis,
            level=level,
            as_index=as_index,
            sort=sort,
            group_keys=group_keys,
            observed=observed,
            dropna=dropna,
        )

    _shared_docs[