Changes all floats in a pandas DataFrame to string

Question

Below I created a function to format all the floats in a pandas DataFrame to a specific precision (6 d.p) and convert to string for output to a GUI (hence why I didn't just change the pandas display options). Is this the most efficient way to convert all floats in a pandas DataFrame to strings of a specified format?

import pandas as pd
df = pd.DataFrame({'x':[5.4525242,6.5254245,7.254542],'y':[5.4525242,6.5254245,7.254542]})
def _format_floats(df):
 df.loc[:,df.dtypes==float]=df.loc[:,df.dtypes==float].apply(lambda row: ["{:.6f}".format(num) for num in row])
_format_floats(df)
#Example output numbers changes to string with 6 decimals
df.iloc[0,0]
#'5.452524'

• AlexV: Welcome to Code Review! Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. Is there anything bothering you? Do you want feedback about style, best practices, or do you need improved performance? See also How to Ask.
• scnerd: Are you sure this code works, as you've posted it here? That df.loc[...] == ... looks like it should just be df.loc[...] = ....

Answer 1

• Since you're already calling .apply, I'd stick with that approach to iteration rather than mix that with a list comprehension.

• It's generally better to avoid making data modifications in-place within a function unless explicitly asked to (via an argument, like inplace=False that you'll see in many Pandas methods) or if it's made clear by the functions name and/or docstring.

• The logic is reasonably complex, so it might be clearer as a named function.

• The leading _ in the function name is usually reserved for "private" functions, whereas this seems to be a general utility function. It's fine if you don't want external code to touch it, that's just not clear from this code snippet.

Here's one way you might re-write the function to follow these tips:

def format_floats(df):
 """Replaces all float columns with string columns formatted to 6 decimal places"""
 def format_column(col):
 if col.dtype != float:
 return col
 return col.apply("{:.6f}".format)
 return df.apply(format_column)

And a usage example:

In [1]: format_floats(pd.DataFrame([{'a': 1, 'b': 2.3}, {'a': 2, 'b': 3.0}]))
Out[1]:
 a b
0 1 2.300000
1 2 3.000000