How to truncate a string using str.format in Python?

Question

How to truncate a string using str.format in Python? Is it even possible?

There is a width parameter mentioned in the Format Specification Mini-Language:

format_spec ::= [[fill]align][sign][#][0][width][,][.precision][type] ... width ::= integer ...

But specifying it apparently only works for padding, not truncating:

>>> '{:5}'.format('aaa') 'aaa ' >>> '{:5}'.format('aaabbbccc') 'aaabbbccc'

So it's more a minimal width than width really.

I know I can slice strings, but the data I process here is completely dynamic, including the format string and the args that go in. I cannot just go and explicitly slice one.

falsetru · Accepted Answer · 2018-08-11 00:14:57Z

120

Use .precision instead:

>>> '{:5.5}'.format('aaabbbccc') 'aaabb'

According to the documentation of the Format Specification Mini-Language:

The precision is a decimal number indicating how many digits should be displayed after the decimal point for a floating point value formatted with 'f' and 'F', or before and after the decimal point for a floating point value formatted with 'g' or 'G'. For non-number types the field indicates the maximum field size - in other words, how many characters will be used from the field content. The precision is not allowed for integer values.

edited Aug 11, 2018 at 0:14

answered Jun 6, 2014 at 7:17

falsetru

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11 Comments

ThorSummoner Over a year ago

Is there a builtin for showing a truncation character as well? Like an ellipsis suffix when the data is truncated like so?

Harvey Over a year ago

Don't forget to give a width and a precision: {:5.5}. This guarantees that the output will always be 5 characters which is probably what OP wanted.

Neal Gokli Over a year ago

@falsetru : Would you be willing to add Harvey's suggestion to your answer?

Hendrik Wiese Over a year ago

Is it possible to truncate the beginning of the string instead of the end?

Shabble Over a year ago

You can make it work for non-strings by forcing them to stringify with !s: e.g. f'{123456789!s:5.5s}' => '12345'

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Dharman · Accepted Answer · 2021-05-15 14:12:06Z

you may truncate by the precision parameter alone:

>>> '{:.1}'.format('aaabbbccc') 'a'

the size parameter is setting the padded size:

>>> '{:3}'.format('ab') ' ab'

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