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Showing content from https://github.com/pandas-dev/pandas/issues/26049 below:

Column dtype change on write of improper value · Issue #26049 · pandas-dev/pandas · GitHub

import pandas as pd
df = pd.DataFrame(columns=['col1'], data=[1,2,3,4], dtype='uint8')
print('original dtypes:')
print(df.dtypes)
print()
print('original data frame:')
print(df)
print()
df.loc[2,'col1']=300
print('dtypes after write operation:')
print(df.dtypes)
print()
print('data frameafter write:')
print(df)

output:

original dtypes:
col1    uint8
dtype: object

original data frame:
   col1
0     1
1     2
2     3
3     4

dtypes after write operation:
col1    int64
dtype: object

data frameafter write:
   col1
0     1
1     2
2    44
3     4

When writing, e.g., a too big integer to an 8-bit unsigned integer column, the value of the written integer is casted to uint8 and the data type of the column is changed to int64.

I would expect that either the value is casted and the data type is retained or the data type gets changed and the value is retained.

original dtypes:
col1    uint8
dtype: object

original data frame:
   col1
0     1
1     2
2     3
3     4

dtypes after write operation:
col1    uint8
dtype: object

data frameafter write:
   col1
0     1
1     2
2    44
3     4

or

original dtypes:
col1    uint8
dtype: object

original data frame:
   col1
0     1
1     2
2     3
3     4

dtypes after write operation:
col1    int64 [or even better uint16]
dtype: object

data frameafter write:
   col1
0     1
1     2
2    300
3     4

commit: None
python: 3.6.8.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.125-linuxkit
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.2
pytest: 4.3.1
pip: 19.0.3
setuptools: 40.8.0
Cython: 0.29.6
numpy: 1.16.2
scipy: 1.2.1
pyarrow: 0.11.1
xarray: 0.11.3
IPython: 7.1.1
sphinx: 2.0.0
patsy: 0.5.1
dateutil: 2.8.0
pytz: 2018.9
blosc: None
bottleneck: None
tables: 3.5.1
numexpr: 2.6.9
feather: None
matplotlib: 3.0.3
openpyxl: None
xlrd: 1.2.0
xlwt: None
xlsxwriter: 1.1.5
lxml.etree: 4.3.0
bs4: 4.7.1
html5lib: None
sqlalchemy: 1.3.1
pymysql: None
psycopg2: 2.7.6.1 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: 0.2.1
pandas_gbq: None
pandas_datareader: None
gcsfs: None

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