What is wrong with this implementation of String interpolation in python*

Question

import re

r = re.compile("#{([^}]*)}")

def I(string):
    def eval_str_match(m):
        return str(eval(m.group(1)))
    return r.sub(eval_str_match,string)

* besides python taste/style/standards

Is there a nicer succinct way to call it then a single letter method?
Is there anything the regex could miss?
should I use repr instead of str ?
I know that eval can be dangerous but I can't see why

I("#{some_func()}\n")

is worse then

"%s\n" % str(some_func())

It's worse because you don't have any error detection from language or IDE. Also if you forgot # user will see actual code. Moreover if you accidentally call 'I' on string that contains input from user, you will have big security hole. There are probably more reasons. — Tomasz Wysocki
– Tomasz Wysocki, Commented Aug 17, 2011 at 5:41
Maybe an ide or a checker but I think python just craps out with an exception either way,no? — Roman A. Taycher
– Roman A. Taycher, Commented Aug 17, 2011 at 5:56
If you have syntax error like "foo)(" Python will raise exception on module-loading time. — Tomasz Wysocki
– Tomasz Wysocki, Commented Aug 17, 2011 at 13:32

agf · Accepted Answer · 2011-08-17 05:28:04Z

2

Not sure exactly what you're trying to accomplish, but does this work?

I = '{}\n'.format
I(some_func())

or

def I(func):
    return "%x\n" % func()
I(some_func())

Using your example from the comment,

I([x*2 for x in [1,2,3]])

works fine (though I don't know what it is you want the output to look like), as does

I(''.join((self.name, ' has ', self.number_of_children)))

but you should really just be doing

'{} has {}'.format(self.name, self.number_of_children)

which is still one line.

edited Aug 17, 2011 at 5:28

answered Aug 17, 2011 at 5:20

agf

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8 Comments

Roman A. Taycher Over a year ago

I'm trying to do ruby style string interpolation

Roman A. Taycher Over a year ago

You can even do crazy stuff like I("#{[x*2 for x in [1,2,3]]}") without multiple lines. But mostly used for stuff like I(" (#{self.name}) has #{self.number_of_children}")

Roman A. Taycher Over a year ago

A simple syntax for putting variables/expressions in strings

agf Over a year ago

Edited to show that stuff. I don't see how Python's syntax is any more complicated.

agf Over a year ago

You can use named replacements if you want more clarity... '{name} has {num_children}'.format(name=self.name, num_children=self.num_children) which looks even more clear than the ruby version to me. Also, what's wrong with self.name + ' has ', str(self.num_children) if you want to keep it simple? Edit: You just commented you prefer this :)

|

Community · Accepted Answer · 2017-05-23 10:32:10Z

This is what I came up with.

in my_print.py:

import sys

def mprint(string='', dictionary=None):
    if dictionary is None:            
        caller = sys._getframe(1)
        dictionary = caller.f_locals
    print string.format(**dictionary)

example:

>>> from my_print import mprint
>>> name = 'Ismael'
>>> mprint('Hi! My name is {name}.')
Hi! My name is Ismael.
>>> new_dict = dict(country='Mars', name='Marvin',
...                 job='space monkey', likes='aliens')
>>> mprint("Hi! My name is {name} and I'm from {country}."
...     " Isn't {name} the best name?!\nDo you know any other {name}?", new_dict)
Hi! My name is Marvin and I'm from Mars. Isn't Marvin the best name?!
Do you know any other Marvin?

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