Hi all,
I would like to modify the standard str.format() in a way that when the
input field is of type str, there is some character replacement, and the
string gets padded or truncated to the given field width. Basically like
this:
fmt = MagicString('<{s:6}>')
print(fmt.format(s='Äußerst'))
Output:
<Aeusse>
I've written a function fix_format() which, given a string and a field width,
does just that. However, I find myself unable to implement a Formatter that
uses this function in the intened way. See the example below, I hope I
sprinkled it with enough comments to make my intents clear. Thanks for any
enlightenment. The interesting part starts somewhere in the middle.
### Self contained example
import re
from string import Formatter
_replacements = [(re.compile(rx), repl) for rx, repl in (\
('Ä', 'Ae'),
('ä', 'ae'),
('Ö', 'Oe'),
('ö', 'oe'),
('Ü', 'Ue'),
('ü', 'ue'),
('ß', 'ss'))]
def fix_format(text, width):
# Seven regex passes seems awfully inefficient. I can't think of a
# better way. Besides the point though.
for rx, repl in _replacements:
text = re.sub(rx, repl, text)
# return truncated / padded version of string
return text[:width] + ' ' * max(0, width - len(text))
class Obj():
"""I'm just an object with some attributes"""
def __init__(self, **kw):
self.__dict__.update(kw)
o = Obj(x="I am X, and I'm too long",
y="ÄÖÜ Ich bin auch zu lang")
z = 'Pad me!'
format_spec = '<{o.x:6}>\n<{o.y:6}>\n<{z:10}>'
# Standard string formatting
print('Standard string formatting:')
print(format_spec.format(o=o, z=z))
# Demonstrate fix_format()
print('\nWanted output:')
print('<' + fix_format(o.x, 6) + '>')
print('<' + fix_format(o.y, 6) + '>')
print('<' + fix_format(z, 10) + '>')
##### This is where my struggle begins. #####
class MagicString(Formatter):
def __init__(self, format_spec):
self.spec = format_spec
super().__init__()
def format(self, **kw):
return(self.vformat(self.spec, [], kw))
def get_field(self, name, a, kw):
# Compound fields have a dot:
obj_name, _, key = name.partition('.')
obj = getattr(kw[obj_name], key) if key else kw[obj_name]
if isinstance(obj, str):
# Here I would like to call fix_format(), but I don't know where
# to get the field width.
print('get_field(): <' + obj + '>')
else:
# Here I'd like to use the "native" formatter of whatever type
# the field is.
pass
return obj, key
def get_value(self, key, a, kw):
'''I don't understand what this method is for, it never gets called'''
raise NotImplementedError
fmt = MagicString(format_spec)
print('\nReal output:')
print(fmt.format(o=o, z=z))
# Weirdly, somewhere on the way the standard formatting kicks in, too, as
# the 'Pad me!' string does get padded (which must be some postprocessing,
# as the string is still unpadded when passed into get_field())
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