Python string format with incorrect values

Question

I have a string that I'd like to format but the values I'm using to format may or may not be proper values (None, or ''). In any event that one of these improper values is passed, I still want the string to format, but ignoring any values that will not work. For example:

mystring = "{:02d}:{:02d}"
mystring.format('', 1)

In this case I'd like my output to be :01, thus negating the fact that '' won't work for the first value in the string. I looked at something like

class Default(dict):
    def __missing__(self, key): 
        return key.join("{}")

d = Default({"foo": "name"})

print("My {foo} is {bar}".format_map(d)) # "My name is {bar}"

But as I'm not using a dictionary for values, I don't think this solution will work for me.

Answer 1

You could write your own formatter and override format_field() to catch these errors and just returns empty strings. Here's the basics (you might want to edit to only catch certain errors):

import string

class myFormat(string.Formatter):
    def format_field(self, value, format_spec):
        try:
            return super().format_field(value, format_spec)
        except ValueError:
            return ''

fmt = myFormat()
mystring = "{:02d}:{:02d}"

print(fmt.format(mystring, *(2, 1)))
# 02:01

print(fmt.format(mystring, *('', 1)))
# :01

Answer 2

You could use a try-except to check if each value is valid:

out = []
for v in ['', 1]:
    # Check if v can be converted to a decimal number
    try:
        out.append(format(v, '02d'))
    except (ValueError, TypeError):
        out.append('')

print(':'.join(out))  # -> :01