Is there a more Pythonic way to pad a string to a variable length using string.format?

Question

I want to pad a string to a certain length, depending on the value of a variable, and I'm wondering if there is a standard, Pythonic way to do this using the string.format mini-language. Right now, I can use string concatenation:

padded_length = 5
print(("\n{:-<" + str((padded_length)) + "}").format("abc"))
# Outputs "abc--"

padded_length = 10
print(("\n{:-<" + str((padded_length)) + "}").format("abc"))
#Outputs "abc-------"

I tried this method:

print(("{:-<{{padded_length}}}".format(padded_length = 10)).format("abc"))

but it raises an IndexError: tuple index out of range exception:

Traceback (most recent call last):
  File "<pyshell#41>", line 1, in <module>
    print(("{:-<{{padded_length}}}".format(padded_length = 10)).format("abc"))
IndexError: tuple index out of range

Is there a standard, in-built way to do this apart from string concatenation? The second method should work, so I'm not sure why it fails.

John La Rooy · Accepted Answer · 2012-07-09 01:40:54Z

5

print(("\n{:-<{}}").format("abc", padded_length))

The other way you were trying, should be written this way

print(("{{:-<{padded_length}}}".format(padded_length=10)).format("abc"))

answered Jul 9, 2012 at 1:40

John La Rooy

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1 Comment

Ricardo Altamirano Over a year ago

Thank you very much. And here was me thinking I had read the documentation thoroughly enough when the answer was quite clear.

octopusgrabbus · Accepted Answer · 2012-07-09 17:27:29Z

5

The following example should provide a solution for you.

padded_length = 5
print("abc".rjust(padded_length, "-"))

prints:

--abc

edited Jul 9, 2012 at 17:27

octopusgrabbus

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answered Jul 9, 2012 at 1:39

Ned Batchelder

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Comments

Joel Cornett · Accepted Answer · 2012-07-09 01:47:33Z

2

You need to escape the outer most curly brackets. The following works fine for me:

>>>'{{0:-<{padded_length}}}'.format(padded_length=10).format('abc')
'abc-------'

answered Jul 9, 2012 at 1:47

Joel Cornett

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1 Comment

Ricardo Altamirano Over a year ago

Ah, that makes sense. I wasn't thinking through which brackets needed to be escaped clearly enough.

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