Padding with single space in Python string format()

Question

I'm afraid this is a basic Python issue, but I couldn't find a proper answer to this. I want to add a single padding space in front of a placeholder.

This is my example:

user = 'Jhon'
'Hi {u}!'.format(u=user if user else '')

# result
'Hi Jhon!'

Now everything is fine until my user var is empty or false, in that case this is the result of above

'Hi !'
#  ^ notice the empty space

Instead I want 'Hi!' as a result.

Now I've tried with format() padding options like {:>1} but ofc is not working since it will add enough padding to reach a character length of 1.

I ended up doing something like this:

'Hi{u:>{p}}!'.format(u=user if user else '', p=1+len(user))

The above works fine, but I think it's kinda hackish and I'd like to now if there is a inbuilt way of doing this that I'm missing.

Thank you!

Avoid the issue: 'Hi {u}!'.format(u=user) if user else 'Hi!' – easy to read and flexible. — Ry-
– Ry- ♦, Commented Mar 12, 2017 at 8:57
Both of you are right indeed, @Ryan suggestion looks quick and clear but unfortunately in my situation I can't use it (the example string was a reduced and simplified case to show my problem). — Gruber
– Gruber, Commented Mar 12, 2017 at 19:32
If @PM2Ring publish his suggestion as an answer I'll mark it as solution — Gruber
– Gruber, Commented Mar 12, 2017 at 19:33

PM 2Ring · Accepted Answer · 2017-03-13 08:57:43Z

1

Here's the solution I posted in the comments:

'Hi{u}!'.format(u=' ' + user if user else '')

The solution you posted in the question doesn't quite work, but it can be fixed by changing your padding length calculation to

p=bool(user) + len(user)

The bool function determines whether it's argument is True-ish or False-ish and returns True or False, respectively. But True and False can be used in arithmetic operations where they evaluate to 1 and 0, respectively.

Here's some code that demonstrates various solutions, including a couple that use literal string interpolation aka f-strings, which are available in Python 3.6+.

for user in ('John', ''):
    print('user is {!r}'.format(user)) 
    # Gruber
    print('Hi{u:>{p}}!'.format(u=user if user else '', p=1+len(user)))
    # Gruber, fixed
    print('Hi{u:>{p}}!'.format(u=user if user else '', p=bool(user) + len(user)))
    # Ryan
    print('Hi {u}!'.format(u=user) if user else 'Hi!')
    # PM 2Ring
    print('Hi{u}!'.format(u=' ' + user if user else ''))
    print('Hi{p}{u}!'.format(u=user, p=' ' if user else ''))
    # Python 3.6
    print(f'Hi{user and " " or ""}{user}!')
    print(f'Hi{" " if user else ""}{user}!')
    # Old `%-style` interpolation
    print('Hi%s%s!' % (user and " " or "", user))

    print()

output

user is 'John'
Hi John!
Hi John!
Hi John!
Hi John!
Hi John!
Hi John!
Hi John!
Hi John!

user is ''
Hi !
Hi!
Hi!
Hi!
Hi!
Hi!
Hi!
Hi!

For info about f-strings please see Formatted string literals in the official Python docs. Also see PEP 498 -- Literal String Interpolation and PEP 536 -- Final Grammar for Literal String Interpolation.

edited Mar 13, 2017 at 8:57

answered Mar 13, 2017 at 6:23

PM 2Ring

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2 Comments

Gruber Over a year ago

Thank you for the very detailed explanation and that nice looking f-string didn't know about it! For anyone interested look this and this

PM 2Ring Over a year ago

@Gruber f-strings are awesome. I generally don't use them in my SO answers since not everyone has 3.6 installed (I installed the alpha version last May). But I guess I should add some links to my answer.

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