DataFrame.loc/iloc fails to update object with new dtype

[TomAugspurger]

Similar to #4312 and #5702, but this seems specific to object dtype -> new dytpe.

In this first case, we likely convert the whole block, even though we just wanted A.

In [20]: df = pd.DataFrame({"A": [1, 2], "B": [3, 4]}, dtype=object)

In [21]: df.dtypes
Out[21]:
A    object
B    object
dtype: object

In [22]: df.loc[:, ['A']] = df.loc[:, ['A']].astype(int)

In [23]: df.dtypes
Out[23]:
A    int64
B    int64
dtype: object

In this one, (maybe a different bug), we fail to convert ['a', 'b'] to float when they start out in an object block with other values.

In [13]: df = pd.DataFrame([[np.nan, np.nan, 1, pd.Timestamp('2000')]], columns=['a', 'b', 'c', 'd'], dtype=object)

In [14]: df.loc[:, ['a', 'b']] = df.loc[:, ['a', 'b']].astype(float)

In [15]: df.dtypes
Out[15]:
a    object
b    object
c    object
d    object
dtype: object

[slnguyen]

Something that might be useful to note is that when indexing with a single label we get the expected output. For example:

df = pd.DataFrame({"A": [1, 2], "B": [3, 4]}, dtype=object)
df.loc[:, 'A'] = (df.loc[:, 'A']).astype(int)

df.dtype returns the expected:

A     int64
B    object

However as you mentioned before:

df = pd.DataFrame({"A": [1, 2], "B": [3, 4]}, dtype=object)
df.loc[:, ['A']] = (df.loc[:, ['A']]).astype(int)

df.dtype returns

A    int64
B    int64
dtype: object

So the issue might have something to do with casting and indexing the dataframe with a list of labels versus single label?

[slnguyen]

I'm not sure if this fix is the right way to resolve the problem, but I've added another control statement so that updating dataframe values corresponding to a list of labels is handled the same way as updating dataframe values corresponding to a single label. The performance test asv_bench/benchmarks/indexing.py returns that the benchmarks are not significantly changed.

pandas/pandas/core/indexing.py

Lines 618 to 652 in 04a0eac

	if isinstance(indexer, tuple):
	indexer = maybe_convert_ix(*indexer)
	# if we are setting on the info axis ONLY
	# set using those methods to avoid block-splitting
	# logic here
	if (len(indexer) > info_axis and
	is_integer(indexer[info_axis]) and
	all(com.is_null_slice(idx)
	for i, idx in enumerate(indexer)
	if i != info_axis) and
	item_labels.is_unique):
	self.obj[item_labels[indexer[info_axis]]] = value
	return
	if isinstance(value, (ABCSeries, dict)):
	# TODO(EA): ExtensionBlock.setitem this causes issues with
	# setting for extensionarrays that store dicts. Need to decide
	# if it's worth supporting that.
	value = self._align_series(indexer, Series(value))
	elif isinstance(value, ABCDataFrame):
	value = self._align_frame(indexer, value)
	if isinstance(value, ABCPanel):
	value = self._align_panel(indexer, value)
	# check for chained assignment
	self.obj._check_is_chained_assignment_possible()
	# actually do the set
	self.obj._consolidate_inplace()
	self.obj._data = self.obj._data.setitem(indexer=indexer,
	value=value)
	self.obj._maybe_update_cacher(clear=True)

Changes were made based on my previous comment. When indexing via single label the code goes through lines 624-631 (see code snipped above). When indexing via a list of labels the code goes through lines 649-652.

[simonjayhawkins]

In this one, (maybe a different bug), we fail to convert ['a', 'b'] to float when they start out in an object block with other values.

on master the first case fails to convert now as well.

>>> import numpy as np
>>>
>>> import pandas as pd
>>>
>>> pd.__version__
'1.1.0.dev0+1029.gbdf969cd6'
>>>
>>> df = pd.DataFrame({"A": [1, 2], "B": [3, 4]}, dtype=object)
>>>
>>> print(df.dtypes)
A    object
B    object
dtype: object
>>> # Out[21]:
>>> # A    object
>>> # B    object
>>> # dtype: object
>>>
>>> df.loc[:, ["A"]] = df.loc[:, ["A"]].astype(int)
>>>
>>> print(df.dtypes)
A    object
B    object
dtype: object
>>> # Out[23]:
>>> # A    int64
>>> # B    int64
>>> # dtype: object
>>>
>>> df = pd.DataFrame(
...     [[np.nan, np.nan, 1, pd.Timestamp("2000")]],
...     columns=["a", "b", "c", "d"],
...     dtype=object,
... )
>>>
>>> df.loc[:, ["a", "b"]] = df.loc[:, ["a", "b"]].astype(float)
>>>
>>> df.dtypes
a    object
b    object
c    object
d    object
dtype: object
>>> # Out[15]:
>>> # a    object
>>> # b    object
>>> # c    object
>>> # d    object
>>> # dtype: object
>>>