Prevent coercion of pandas data frames while indexing and inserting rows

Question

I'm working with individual rows of pandas data frames, but I'm stumbling over coercion issues while indexing and inserting rows. Pandas seems to always want to coerce from a mixed int/float to all-float types, and I can't see any obvious controls on this behaviour.

For example, here is a simple data frame with a as int and b as float:

import pandas as pd
pd.__version__  # '0.25.2'

df = pd.DataFrame({'a': [1], 'b': [2.2]})
print(df)
#    a    b
# 0  1  2.2
print(df.dtypes)
# a      int64
# b    float64
# dtype: object

Here is a coercion issue while indexing one row:

print(df.loc[0])
# a    1.0
# b    2.2
# Name: 0, dtype: float64
print(dict(df.loc[0]))
# {'a': 1.0, 'b': 2.2}

And here is a coercion issue while inserting one row:

df.loc[1] = {'a': 5, 'b': 4.4}
print(df)
#      a    b
# 0  1.0  2.2
# 1  5.0  4.4
print(df.dtypes)
# a    float64
# b    float64
# dtype: object

In both instances, I want the a column to remain as an integer type, rather than being coerced to a float type.

Answer 1

After some digging, here are some terribly ugly workarounds. (A better answer will be accepted.)

A quirk found here is that non-numeric columns stops coercion, so here is how to index one row to a dict:

dict(df.assign(_='').loc[0].drop('_', axis=0))
# {'a': 1, 'b': 2.2}

And inserting a row can be done by creating a new data frame with one row:

df = df.append(pd.DataFrame({'a': 5, 'b': 4.4}, index=[1]))
print(df)
#    a    b
# 0  1  2.2
# 1  5  4.4

Both of these tricks are not optimised for large data frames, so I would greatly appreciate a better answer!

Answer 2

Whenever you are getting data from dataframe or appending data to a dataframe and need to keep the data type same, avoid conversion to other internal structures which are not aware of the data types needed.

When you do df.loc[0] it converts to pd.Series,

>>> type(df.loc[0])
<class 'pandas.core.series.Series'>

And now, Series will only have a single dtype. Thus coercing int to float.

Instead keep structure as pd.DataFrame,

>>> type(df.loc[[0]])
<class 'pandas.core.frame.DataFrame'>

Select row needed as a frame and then convert to dict

>>> df.loc[[0]].to_dict(orient='records')
[{'a': 1, 'b': 2.2}]

Similarly, to add a new row, Use pandas pd.DataFrame.append function,

>>> df = df.append([{'a': 5, 'b': 4.4}]) # NOTE: To append as a row, use []
   a    b
0  1  2.2
0  5  4.4

The above will not cause type conversion,

>>> df.dtypes
a      int64
b    float64
dtype: object

Answer 3

The root of the problem is that

1. The indexing of pandas dataframe returns a pandas series

We can see that:

type(df.loc[0])
# pandas.core.series.Series

And a series can only have one dtype, in your case either int64 or float64.

There are two workarounds come to my head:

print(df.loc[[0]])
# this will return a dataframe instead of series
# so the result will be
#    a    b
# 0  1  2.2

# but the dictionary is hard to read
print(dict(df.loc[[0]]))
# {'a': 0    1
# Name: a, dtype: int64, 'b': 0    2.2
# Name: b, dtype: float64}

or

print(df.astype(object).loc[0])
# this will change the type of value to object first and then print
# so the result will be
# a      1
# b    2.2
# Name: 0, dtype: object

print(dict(df.astype(object).loc[0]))
# in this way the dictionary is as expected
# {'a': 1, 'b': 2.2}

2. When you append a dictionary to a dataframe, it will convert the dictionary to a Series first and then append. (So the same problem happens again)

https://github.com/pandas-dev/pandas/blob/master/pandas/core/frame.py#L6973

if isinstance(other, dict):
    other = Series(other)

So your walkaround is actually a solid one, or else we could:

df.append(pd.Series({'a': 5, 'b': 4.4}, dtype=object, name=1))
#    a    b
# 0  1  2.2
# 1  5  4.4