将值设置为数据框时，熊猫会更改日期时间

Question

When I use iloc to set a datetime value to a dataframe cell, the value is changed

import datetime as dt
import pandas as pd

t = '2019-12-29 13:17:34.678000'
t = dt.datetime.fromisoformat(t)
print(t)
# 2019-12-29 13:17:34.678000

df = pd.DataFrame({'t': [t]})
print(df)
#                         t
# 0 2019-12-29 13:17:34.678

df.iloc[0, df.columns.get_loc('t')] = t
print(df)
#                               t
# 0 2019-12-29 13:17:34.678000128

Why is this happening and how can I prevent it?

Answer 1

You can apply function isoformat that will trunc time to milliseconds removing microsecond precision:

t = dt.datetime.fromisoformat(t).isoformat(timespec='milliseconds')

For details regarding isoformat, please follow: https://docs.python.org/3/library/datetime.html#datetime.time.microsecond

Please note that as documentation says, excluded time components are truncated, not rounded.

Answer 2

This seems to be a bug in pandas 0.25.2 . It doesn't happen in the latest version, 1.0.1 .

In 0.25.2 it can be worked around by using .at instead of .iloc

t = '2019-12-29 13:17:34.678000'
t = dt.datetime.fromisoformat(t)
print(t)
# 2019-12-29 13:17:34.678000

df = pd.DataFrame({'t': [t]})
print(df)
#                         t
# 0 2019-12-29 13:17:34.678

df.at[0, 't'] = t
print(df)
#                         t
# 0 2019-12-29 13:17:34.678