I have integer numbers in the range of 0 up until (including) 100. I want to convert them to strings of a fixed length of 3 with space padding and alignment to the right.
I’ve tried to use the following format string, but it adds another space for three digit numbers, which makes them length 4 instead of 3.
fmt = lambda x: "{: 3d}".format(x)
[fmt(99), fmt(100)] # produces [' 99', ' 100'] instead of [' 99', '100']
Interestingly, it works as expected when zero-padding is used:
fmt = lambda x: "{:03d}".format(x)
[fmt(99), fmt(100)] # produces ['099', '100'] as expected
Why is this? How can I fix this?
Do I really need to convert to string first?
fmt = lambda x: "{:>3s}".format(str(x))
[fmt(99), fmt(100)] # produces [' 99', '100'] as expected
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Answer
By default, numbers are aligned to the right and padded with spaces when formatted, so you should just specify the width:
>>> '{:3d}'.format(99)
' 99'
>>> '{:3d}'.format(100)
'100'
Alternatively, you can specify both the fill character and alignment:
>>> '{: >3d}'.format(99)
' 99'
>>> '{: >3d}'.format(100)
'100'
A single space before width, however, is treated as the sign option. Quoting the documentation:
The sign option is only valid for number types, and can be one of the following:
'+'indicates that a sign should be used for both positive as well as negative numbers.
'-'indicates that a sign should be used only for negative numbers (this is the default behavior).
' 'indicates that a leading space should be used on positive numbers, and a minus sign on negative numbers.
That’s why "{: 3d}" formats with a leading space in your examples.