Python字符串格式 - 限制字符串长度，但修剪字符串开头

Question

I'm using Python standard logging module with custom formatter where I limit length of some fields. It uses standard % Python operator.

I can apply limit for percent-formatted string like this (this limits length to 10 chars):

>>> "%.10s" % "Lorem Ipsum"
'Lorem Ipsu'

Is it possible to trim it from the beginning, so the output is 'orem Ipsum' ( without manipulating right-side argument )?

Answer 1

This can easily be done through slicing, so you do not require any string format manipulation to do your JOB

>>> "Lorem Ipsum"[-10:]
'orem Ipsum'

Answer 2

Is it possible to trim it from the beginning with % formatting?

Python's % formatting comes from C's printf .

Note that the . indicates precision for a float. That it works on a string is a mere side effect, and unfortunately, there is no provision in the string formatting specification to accommodate stripping a string from the left to a fixed max width.

Therefore if you must strip a string to a fixed width from the end, I recommend to slice from a negative index. This operation is robust, and won't fail if the string is less than 10 chars.

>>> up_to_last_10_slice = slice(-10, None)
>>> 'Lorem Ipsum'[up_to_last_10_slice]
'orem Ipsum'
>>> 'Ipsum'[up_to_last_10_slice]
'Ipsum'

str.format also no help

str.format is of no help here, the width is a minimum width:

>>> '{lorem:>10}'.format(lorem='Lorem Ipsum')
'Lorem Ipsum'
>>> '{lorem:*>10}'.format(lorem='Lorem')
'*****Lorem'

(The asterisk, " * ", is the fill character.)

Answer 3

I had the same question and came up with this solution using LogRecordFactory.

orig_factory = logging.getLogRecordFactory()

def record_factory(*args, **kwargs):
    record = orig_factory(*args, **kwargs)
    record.sname = record.name[-10:] if len(
        record.name) > 10 else record.name
    return record

logging.setLogRecordFactory(record_factory)

Here I am truncating the name to 10 characters and storing it in the attribute sname, which can be used as any other value.

%(sname)10s

It is possible to store the truncated name in record.name, but I wanted to keep the original name around too.