How to implement string interpolation in Python

String interpolation is a process of substituting values of local variables into placeholders in a string.

It is implemented in many programming languages such as Scala:

//Scala 2.10+
var name = "John";
println(s"My name is $name")
>>> My name is John

Perl:

my $name = "John";
print "My name is $name";
>>> My name is John

CoffeeScript:

name = "John"
console.log "My name is #{name}"
>>> My name is John

and many others.

We have string interpolation in Python since 3.6, but let’s take a look at how we could implement our own string interpolation is just two lines of Python code!

But let’s start with basics. An idiomatic way to build a complex string in Python, before 3.6, is to use the “format” function:

print "Hi, I am {} and I am {} years old".format(name, age)
>>> Hi, I am John and I am 26 years old

Which is much cleaner than using string concatenation:

print "Hi, I am " + name + " and I am " + str(age) + " years old"
Hi, I am John and I am 26 years old

But if you use the format function in this way the output depends on the order of arguments:

print "Hi, I am {} and I am {} years old".format(age, name)
Hi, I am 26 and I am John years old

To avoid that we can pass key-value arguments to the “format” function:

print "Hi, I am {name} and I am {age} years old".format(name="John", age=26)
Hi, I am John and I am 26 years old

print "Hi, I am {name} and I am {age} years old".format(age=26, name="John")
Hi, I am John and I am 26 years old

Here we had to pass all variables for string interpolation to the “format” function, but still we have not achieved what we wanted, because “name” and “age” are not local variables. Can “format” somehow access local variables?

To do it we can get a dictionary with all local variables using the locals function:

name = "John"
age = 26

locals()
>>> {
 ...
 'age': 26,
 'name': 'John',
 ...
}

Now we just need to somehow pass it to the format function. Unfortunately we cannot just call it as s.format(locals()):

print "Hi, I am {name} and I am {age} years old".format(locals())
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-5-0fb983071eb8> in <module>()
----> 1 print "Hi, I am {name} and I am {age} years old".format(locals())

KeyError: 'name'

This is because locals returns a dictionary, while format expects key-value parameters.

Luckily we can convert a dictionary into key-value parameters using \*\* opeartor. If we have a function that expects key-value arguments:

def foo(arg1=None, arg2=None):
    print "arg1 = " + str(arg1)
    print "arg2 = " + str(arg2)

We can pass parameters packed in a dictionary:

d = {
    'arg1': 1,
    'arg2': 42
}

foo(**d)
>>> arg1 = 1
arg2 = 42

Now we can use this technique to implement our first version of string interpolation:

print "Hi, I am {name} and I am {age} years old".format(**locals())
Hi, I am John and I am 26 years old

It works, but looks cumbersome. With this approach every time we need to interpolate our string we would need to write format(\*\*locals()).
It would be great if we could write a function that would interpolate a string like this:

# Can we implement inter() function in Python?
print inter("Hi, I am {name} and I am {age} years old")
>>> Hi, I am John and I am 26 years old

At first it seems impossible, since if we move interpolation code to another function it would not be able to access local variables from a scope where it was called from:

name = "John"
print inter("My name is {name}")

...

def inter(s):
  # How can we access "name" variable from here?
  return s.format(...)

And yet, it is possible. Python provides a way to inspect current stack with the sys.\_getframe function:

import sys

def foo():
     foo_var = 'foo'
     bar()


 def bar():
     # sys._getframe(0) would return frame for function "bar"
     # so we need to to access 1-st frame
     # to get local variables from "foo" function
     previous_frame = sys._getframe(1)
     previous_frame_locals = previous_frame.f_locals
     print previous_frame_locals['foo_var']


foo()
>>> foo

So the only thing that is left is to combine frames introspection with the format function. Here are 2 lines of code that would do the trick:

def inter(s):
    return s.format(**sys._getframe(1).f_locals)

name = "John"
age = 26

print inter("Hi, I am {name} and I am {age} years old")
>>> Hi, I am John and I am 26 years old