Issue
Related:
You use curly braces ( {•••} ) to denote parts of strings to be formatted. If you want to use literal curly brace characters so that they're ignored by .format(), you use double curly braces ( {{•••}} ). MCVE:
string = "{format} {{This part won't be formatted. The final string will have literal curly braces here.}}"
print string.format(format='string')
If you have a chain of .format()s, you double the number of braces for each time you use .format(). The last one is surrounded by 4 braces and ends up with literal curly braces in the final output. MCVE:
string = "{format1} {{format2}} {{{{3rd one won't be formatted. The final string will have literal curly braces here.}}}}"
print string.format(format1='string1').format(format2='string2')
It's also possible to format another format string into a format string. The last one is surrounded by 4 braces and ends up with literal curly braces in the final output. MCVE:
string = "{format1} {{{{3rd one won't be formatted. The final string will have literal curly braces here.}}}}"
print string.format(format1='{format2}').format(format2='string')
The problem arises when you use a chain of .format()s depending on conditions determined at runtime. If you want a set of the curly braces to be escaped as literal characters, how many do you use? MCVE:
string = "{} {{{{{{Here I don't know exactly how many curly braces to use because this string is formatted differently due to conditions that I have no control over.}}}}}}"
if fooCondition:
string = string.format('{} bar')
if barCondition:
string = string.format('{} baz')
if bazCondition:
string = string.format('{} buzz')
string = string.format('foo')
print string
The 1st part of the string has 4 possible outputs:
foofoo barfoo baz barfoo buzz baz bar
The 2nd part of the string ends up with a different number of curly braces depending on how many conditions are True. I want the 2nd part's curly braces to stay permanently escaped, like not "shed a layer" every time .format() is called. I can solve the problem like this, MCVE:
string = "{} {{DRY - Don't repeat yourself!}}"
if fooCondition:
string = string.format('{} bar').replace("{DRY - Don't repeat yourself!}", "{{DRY - Don't repeat yourself!}}")
if barCondition:
string = string.format('{} baz').replace("{DRY - Don't repeat yourself!}", "{{DRY - Don't repeat yourself!}}")
if bazCondition:
string = string.format('{} buzz').replace("{DRY - Don't repeat yourself!}", "{{DRY - Don't repeat yourself!}}")
string = string.format('foo')
print string
But that's duplicate code (bad practice).
The MCVEs aren't my real code. My real code runs on a Google App Engine web server. It's super long and complex. I'm working with HTML, CSS, and JavaScript in strings. I want to insert content into the HTML via .format() without messing up the curly braces of CSS and JS. My current implementation is un-scalable and very error-prone. I have to manage up to 5 consecutive curly braces (like this: {{{{{•••}}}}} ) to pass through .format() chains untouched. I need to periodically re-insert curly braces into strings that aren't formatted a fixed number of times. What's an elegant way to fix this spaghetti code?
How to PERMANENTLY escape curly braces in Python format string?
Solution
I put together a partialformat function (in python3.x) that overrides the string format method to allow you to format only those sections of the string that require formatting. edit: I've included a python 2 version as well.
## python 3x version
import string
from _string import formatter_field_name_split
################################################################################
def partialformat(s: str, recursionlimit: int = 10, **kwargs):
"""
vformat does the acutal work of formatting strings. _vformat is the
internal call to vformat and has the ability to alter the recursion
limit of how many embedded curly braces to handle. But for some reason
vformat does not. vformat also sets the limit to 2!
The 2nd argument of _vformat 'args' allows us to pass in a string which
contains an empty curly brace set and ignore them.
"""
class FormatPlaceholder(object):
def __init__(self, key):
self.key = key
def __format__(self, spec):
result = self.key
if spec:
result += ":" + spec
return "{" + result + "}"
def __getitem__(self, item):
return
class FormatDict(dict):
def __missing__(self, key):
return FormatPlaceholder(key)
class PartialFormatter(string.Formatter):
def get_field(self, field_name, args, kwargs):
try:
obj, first = super(PartialFormatter, self).get_field(field_name, args, kwargs)
except (IndexError, KeyError, AttributeError):
first, rest = formatter_field_name_split(field_name)
obj = '{' + field_name + '}'
# loop through the rest of the field_name, doing
# getattr or getitem as needed
for is_attr, i in rest:
if is_attr:
try:
obj = getattr(obj, i)
except AttributeError as exc:
pass
else:
obj = obj[i]
return obj, first
fmttr = PartialFormatter()
try:
fs, _ = fmttr._vformat(s, ("{}",), FormatDict(**kwargs), set(), recursionlimit)
except Exception as exc:
raise exc
return fs
edit: looks like python 2.x has some minor differences.
## python 2.x version
import string
formatter_field_name_split = str._formatter_field_name_split
def partialformat(s, recursionlimit = 10, **kwargs):
"""
vformat does the acutal work of formatting strings. _vformat is the
internal call to vformat and has the ability to alter the recursion
limit of how many embedded curly braces to handle. But for some reason
vformat does not. vformat also sets the limit to 2!
The 2nd argument of _vformat 'args' allows us to pass in a string which
contains an empty curly brace set and ignore them.
"""
class FormatPlaceholder(object):
def __init__(self, key):
self.key = key
def __format__(self, spec):
result = self.key
if spec:
result += ":" + spec
return "{" + result + "}"
def __getitem__(self, item):
return
class FormatDict(dict):
def __missing__(self, key):
return FormatPlaceholder(key)
class PartialFormatter(string.Formatter):
def get_field(self, field_name, args, kwargs):
try:
obj, first = super(PartialFormatter, self).get_field(field_name, args, kwargs)
except (IndexError, KeyError, AttributeError):
first, rest = formatter_field_name_split(field_name)
obj = '{' + field_name + '}'
# loop through the rest of the field_name, doing
# getattr or getitem as needed
for is_attr, i in rest:
if is_attr:
try:
obj = getattr(obj, i)
except AttributeError as exc:
pass
else:
obj = obj[i]
return obj, first
fmttr = PartialFormatter()
try:
fs = fmttr._vformat(s, ("{}",), FormatDict(**kwargs), set(), recursionlimit)
except Exception as exc:
raise exc
return fs
Usage:
class ColorObj(object):
blue = "^BLUE^"
s = '{"a": {"b": {"c": {"d" : {} {foo:<12} & {foo!r} {arg} {color.blue:<10} {color.pink} {blah.atr} }}}}'
print(partialformat(s, foo="Fooolery", arg="ARRrrrrrg!", color=ColorObj))
Output:
{"a": {"b": {"c": {"d" : {} Fooolery & 'Fooolery' Fooolery ARRrrrrrg! ^BLUE^ {color.pink} {blah.atr} }}}}
Answered By - Marcel Wilson
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