Python - Turn all items in a Dataframe to strings

Question

I followed the following procedure: In Python, how do I convert all of the items in a list to floats? because each column of my Dataframe is list, but instead of floats I chose to change all the values to strings.

df = [str(i) for i in df]

But this failed.

It simply erased all the data except for the first row of column names.

Then, trying df = [str(i) for i in df.values] resulted in changing the entire Dataframe into one big list, but that messes up the data too much to be able to meet the goal of my script which is to export the Dataframe to my Oracle table.

Is there a way to convert all the items that are in my Dataframe that are NOT strings into strings?

Answer 1

With pandas >= 1.0 there is now a dedicated string datatype:

You can convert your column to this pandas string datatype using .astype('string'):

df = df.astype('string')

This is different from using str which sets the pandas 'object' datatype:

df = df.astype(str)

You can see the difference in datatypes when you look at the info of the dataframe:

df = pd.DataFrame({
    'zipcode_str': [90210, 90211] ,
    'zipcode_string': [90210, 90211],
})

df['zipcode_str'] = df['zipcode_str'].astype(str)
df['zipcode_string'] = df['zipcode_str'].astype('string')

df.info()

# you can see that the first column has dtype object
# while the second column has the new dtype string
 #   Column          Non-Null Count  Dtype 
---  ------          --------------  ----- 
 0   zipcode_str     2 non-null      object
 1   zipcode_string  2 non-null      string
dtypes: object(1), string(1)

From the docs:

The 'string' extension type solves several issues with object-dtype NumPy arrays:

1) You can accidentally store a mixture of strings and non-strings in an object dtype array. A StringArray can only store strings.

2) object dtype breaks dtype-specific operations like DataFrame.select_dtypes(). There isn’t a clear way to select just text while excluding non-text, but still object-dtype columns.

3) When reading code, the contents of an object dtype array is less clear than string.

Information about pandas 1.0 can be found here:
https://pandas.pydata.org/pandas-docs/version/1.0.0/whatsnew/v1.0.0.html

Answer 2

You can use this:

df = df.astype(str)

out of curiosity I decided to see if there is any difference in efficiency between the accepted solution and mine.

The results are below:

example df:

df = pd.DataFrame([list(range(1000))], index=[0])

test df.astype:

%timeit df.astype(str) 
>> 100 loops, best of 3: 2.18 ms per loop

test df.applymap:

%timeit df.applymap(str)
1 loops, best of 3: 245 ms per loop

It seems df.astype is quite a lot faster :)

Answer 3

You can use applymap method:

df = df.applymap(str)

Answer 4

This worked for me:

dt.applymap(lambda x: x[0] if type(x) is list else None)