Dataframe column dtype changed from int8 to int64 when setting complete column · Issue #11638 · pandas-dev/pandas (original) (raw)

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Description

The following example should explain:

Python 2.7.10 |Continuum Analytics, Inc.| (default, Oct 19 2015, 18:04:42) Type "copyright", "credits" or "license" for more information.

IPython 4.0.0 -- An enhanced Interactive Python. ? -> Introduction and overview of IPython's features. %quickref -> Quick reference. help -> Python's own help system. object? -> Details about 'object', use 'object??' for extra details.

In [1]: import numpy as np

In [2]: import pandas as pd

In [3]: pd.show_versions()

INSTALLED VERSIONS

commit: None python: 2.7.10.final.0 python-bits: 64 OS: Linux OS-release: 3.13.0-52-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8

pandas: 0.17.0 nose: 1.3.7 pip: 7.1.2 setuptools: 18.4 Cython: None numpy: 1.10.1 scipy: 0.16.0 statsmodels: 0.6.1 IPython: 4.0.0 sphinx: None patsy: 0.4.0 dateutil: 2.4.2 pytz: 2015.7 blosc: None bottleneck: None tables: None numexpr: None matplotlib: 1.4.3 openpyxl: None xlrd: None xlwt: None xlsxwriter: None lxml: None bs4: None html5lib: None httplib2: None apiclient: None sqlalchemy: None pymysql: None psycopg2: None

In [4]: df = pd.DataFrame({'one': np.full(10, 0, dtype=np.int8)})

In [5]: df Out[5]: one 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

In [6]: df.dtypes Out[6]: one int8 dtype: object

In [7]: df.loc[1, 'one'] = 6

In [8]: df Out[8]: one 0 0 1 6 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

In [9]: df.dtypes Out[9]: one int8 dtype: object

In [10]: df.one = np.int8(7)

In [11]: df.dtypes Out[11]: one int64 dtype: object

In [12]: df Out[12]: one 0 7 1 7 2 7 3 7 4 7 5 7 6 7 7 7 8 7 9 7

So it is cast to the correct dtype if a slice of the column is changed but setting the whole column changes the dtype even when explicitly set to np.int8