To make all your floats show comma separators by default in pandas versions 0.23 through 0.25 set the following:

pd.options.display.float_format = '{:,}'.format

https://pandas.pydata.org/pandas-docs/version/0.23.4/options.html

In pandas version 1.0 this leads to some strange formatting in some cases.

you are converting to string each element : "{:,}".format(x)

but I guess you want to display your numbers in your pandas DataFrame to show comma separators by default, for this you can do it but for float data type:

pd.options.display.float_format = '{:,}'.format

if you want also for int type you should monkey-patch pandas.io.formats.format.IntArrayFormatter .

You could monkey-patch pandas.io.formats.format.IntArrayFormatter:

import contextlib
import numpy as np
import pandas as pd
import pandas.io.formats.format as pf
np.random.seed(2015)

@contextlib.contextmanager
def custom_formatting():
    orig_float_format = pd.options.display.float_format
    orig_int_format = pf.IntArrayFormatter

    pd.options.display.float_format = '{:0,.2f}'.format
    class IntArrayFormatter(pf.GenericArrayFormatter):
        def _format_strings(self):
            formatter = self.formatter or '{:,d}'.format
            fmt_values = [formatter(x) for x in self.values]
            return fmt_values
    pf.IntArrayFormatter = IntArrayFormatter
    yield
    pd.options.display.float_format = orig_float_format
    pf.IntArrayFormatter = orig_int_format


df = pd.DataFrame(np.random.randint(10000, size=(5,3)), columns=list('ABC'))
df['D'] = np.random.random(df.shape[0])*10000

with custom_formatting():
    print(df)

yields

      A     B     C        D
0 2,658 2,828 4,540 8,961.77
1 9,506 2,734 9,805 2,221.86
2 3,765 4,152 4,583 2,011.82
3 5,244 5,395 7,485 8,656.08
4 9,107 6,033 5,998 2,942.53

while outside of the with-statement:

print(df)

yields

      A     B     C            D
0  2658  2828  4540  8961.765260
1  9506  2734  9805  2221.864779
2  3765  4152  4583  2011.823701
3  5244  5395  7485  8656.075610
4  9107  6033  5998  2942.530551

You can format your column by doing this:

df['new_column_name'] = df['column_name'].map('{:,.2f}'.format)

But keep in mind that the new column will contain strings, not floats.