Question

I have the following Python 2.x code, which generates a header row for tabular data:

headers = ['Name', 'Date', 'Age']
maxColumnWidth = 20 # this is just a placeholder
headerRow = "|".join( ["%s" % k.center(maxColumnWidth) for k in headers] )
print(headerRow)

This code outputs the following:

         Name          |        Date        |          Age         

Which is exactly what I want – the data is nicely formatted and centered in columns of width maxColumnWidth. (maxColumnWidth is calculated earlier in the program)

According to the Python docs, you should be able to do the same thing in Python3 with curly brace string formatting, as follows:

headerRow = "|".join( ["{:^maxColumnWidth}".format(k) for k in headers] )

However, when I do this, I get the following:

ValueError: Invalid conversion specification

But, if I do this:

headerRow = "|".join( ["{:^30}".format(k) for k in headers] )

Everything works fine.
My question is: How do I use a variable in the format string instead of an integer?:

headerRow = "|".join( ["{:^maxColumnWidth}".format(k) for k in headers] )

Answer 1

headers = ['Name', 'Date', 'Age']
maxColumnWidth=21
headerRow = "|".join( "{k:^{m}}".format(k=k,m=maxColumnWidth) for k in headers )
print(headerRow)

yields

        Name        |        Date        |        Age         

1. You can represent the width maxColumnWidth as {m}, and then
   substitute the value through a format parameter.
2. No need to use brackets (list comprehension) inside the join. A
   generator expression (without brackets) suffices.