Pandas: how to identify columns with dtype object but mixed-type items?

Question

In a pandas dataframe, a column with dtype = object can, in fact, contain items of mixed types, eg integers and strings.

In this example, column a is dtype object, but the first item is string while all the others are int:

import numpy as np, pandas as pd
df=pd.DataFrame()
df['a']=np.arange(0,9)
df.iloc[0,0]='test'
print(df.dtypes)
print(type(df.iloc[0,0]))
print(type(df.iloc[1,0]))

My question is: is there a quick way to identify which columns with dtype=object contain, in fact, mixed types like above? Since pandas does not have a dtype = str, this is not immediately apparent.

However, I have had situations where, importing a large csv file into pandas, I would get a warning like:

sys:1: DtypeWarning: Columns (15,16) have mixed types. Specify dtype option on import or set low_memory=False

Is there an easy way to replicate that and explicitly list the columns with mixed types? Or do I manually have to go through them one by one, see if I can convert them to string, etc?

The background is that I am trying to export a dataframe to a Microsoft SQL Server using DataFrame.to_sql and SQLAlchemy. I get an

OverflowError: int too big to convert

but my dataframe does not contain columns with dtype int - only object and float64. I'm guessing this is because one of the object columns must have both strings and integers.

Answer 1

Setup

df = pd.DataFrame(np.ones((3, 3)), columns=list('WXY')).assign(Z='c')
df.iloc[0, 0] = 'a'
df.iloc[1, 2] = 'b'

df

   W    X  Y  Z
0  a  1.0  1  c
1  1  1.0  b  c
2  1  1.0  1  c

---

Solution
Find all types and count how many unique ones per column.

df.loc[:, df.applymap(type).nunique().gt(1)]

   W  Y
0  a  1
1  1  b
2  1  1