BUG: astype() on an integer DataFrame changes the order of data

[wjsi]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

---

Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

import numpy as np
import pandas as pd
npa = np.random.RandomState(0).randint(1000, size=(20, 8))
df = pd.DataFrame(npa, columns=[f'c{i}' for i in range(8)])
print(df.iloc[:6, :3])
#     c0   c1   c2
# 0  684  559  629
# 1    9  723  277
# 2  600  396  314
# 3  600  849  677
# 4  115  976  755
# 5   99  984  177
print(df.iloc[:6, :3].astype('int32'))
#     c0   c1   c2
# 0  684  600  115
# 1  559  396  976
# 2  629  314  755
# 3    9  600   99
# 4  723  849  984
# 5  277  677  177

Problem description

The order of output result is changed unexpectedly after astype is called. It may caused by data continuity as df.iloc[:6, :3].copy().astype('int32') works as expected.

Expected Output

print(df.iloc[:6, :3].astype('int32'))
#     c0   c1   c2
# 0  684  559  629
# 1    9  723  277
# 2  600  396  314
# 3  600  849  677
# 4  115  976  755
# 5   99  984  177

Output of pd.show_versions()

INSTALLED VERSIONS

commit : f00ed8f
python : 3.8.5.final.0
python-bits : 64
OS : Darwin
OS-release : 19.6.0
Version : Darwin Kernel Version 19.6.0: Thu May 6 00:48:39 PDT 2021; root:xnu-6153.141.33~1/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : None
LOCALE : zh_CN.UTF-8
pandas : 1.3.0
numpy : 1.18.5
pytz : 2021.1
dateutil : 2.8.1
pip : 20.2.2
setuptools : 49.6.0.post20200814
Cython : 0.29.23
pytest : 6.2.2
hypothesis : 6.3.0
sphinx : 3.2.1
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.17.0
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : 0.4.2
gcsfs : None
matplotlib : 3.2.2
numexpr : 2.7.1
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 4.0.0
pyxlsb : None
s3fs : None
scipy : 1.6.0
sqlalchemy : 1.3.18
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : 0.53.0

[rhshadrach]

Thanks for the report - seeing this on master but not 1.2.x.

[debnathshoham]

take

[debnathshoham]

This appears to be a 'deeper' issue.

So, the dataframe is converted to a BlockManager. And then to ndarray (with both c_contiguous and f_contiguous flags False). And then finally during reshape, it is reshaped with order = 'C'. If we use order = 'F', the reshape works as expected, but breaks test pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean for dataframes.

[mzeitlin11]

A bisect indicates the cause as #38507, xref #38592 also involved that pr