pandas.errors.DtypeWarning

exception pandas.errors.DtypeWarning

Warning raised when reading different dtypes in a column from a file.

Raised for a dtype incompatibility. This can happen whenever read_csv or read_table encounter non-uniform dtypes in a column(s) of a given CSV file.

See also

read_csv

Read CSV (comma-separated) file into a DataFrame.

read_table

Read general delimited file into a DataFrame.

Notes

This warning is issued when dealing with larger files because the dtype checking happens per chunk read.

Despite the warning, the CSV file is read with mixed types in a single column which will be an object type. See the examples below to better understand this issue.

Examples

This example creates and reads a large CSV file with a column that contains int and str.

>>> df = pd.DataFrame({'a': (['1'] * 100000 + ['X'] * 100000 +
...                          ['1'] * 100000),
...                    'b': ['b'] * 300000})
>>> df.to_csv('test.csv', index=False)
>>> df2 = pd.read_csv('test.csv')
... # DtypeWarning: Columns (0) have mixed types

Important to notice that df2 will contain both str and int for the same input, ‘1’.

>>> df2.iloc[262140, 0]
'1'
>>> type(df2.iloc[262140, 0])
<class 'str'>
>>> df2.iloc[262150, 0]
1
>>> type(df2.iloc[262150, 0])
<class 'int'>

One way to solve this issue is using the dtype parameter in the read_csv and read_table functions to explicit the conversion:

>>> df2 = pd.read_csv('test.csv', sep=',', dtype={'a': str})

No warning was issued.

>>> import os
>>> os.remove('test.csv')