Question

i’m trying to do some “post”/”lazy” evaluation of arguments on my strings. Suppose i’ve this:

s = "SELECT * FROM {table_name} WHERE {condition}" 

I’d like to return the string with the {table_name} replaced, but not the {condition}, so, something like this:

s1 = s.format(table_name = "users")

So, I can build the hole string later, like:

final = s1.format(condition= "user.id = {id}".format(id=2))

The result should be, of course:

"SELECT * FROM users WHERE user.id = 2" 

I’ve found this previous answer, this is exactly what I need, but i’d like to use the format string function.

python, format string
Thank you for your help!

Answer 1

You can’t use the format function because it will raise a KeyError.

string.Template supports safe substituion:

from string import Template
s = Template('SELECT * FROM $table_name WHERE $condition')
s.safe_substitute(table_name='users')

'SELECT * FROM users WHERE $condition'

If you use plain variable names (no format specifiers, no indexing, etc..) this will also work (thanks @Simeon Visser for the idea):

def myformat(s, *args, **kwargs):
  while True:
    try:
      return s.format(*args, **kwargs)
    except KeyError as e:
      e=e.args[0]
      kwargs[e] = "{%s}" % e

s = "SELECT * FROM {table_name} WHERE {condition}" 
myformat(s, table_name="users")

'SELECT * FROM users WHERE {condition}'