Python String Interpolation

I code in atleast 3-4 programming languages regularly, and I often mix up Ruby, Python, and JavaScript syntax. To help keep things organized and provide a quick reference, I’m planning a series of articles that outline common functionalities in these languages.

In this article, I’ll cover string interpolation in Python. Interestingly, there are five different ways to achieve this in Python. Not only do I need to remember syntax across multiple languages, but I also have to remember five different methods for the same task in Python. Haha.

Here are the five ways to do string interpolation in Python:

Using % Operator

name = "Alice"
age = 30
greeting = "Hello, %s. You are %d years old." % (name, age)
print(greeting)

I like this method because it’s reminiscent of C, easy to remember, and doesn’t require many characters.

Using str.format() Method with Positions

name = "Alice"
age = 30
greeting = "Hello, {}. You are {} years old.".format(name, age)
print(greeting)

This is similar to the % operator but more verbose.

Using str.format() Method with Variable Names

greeting = "Hello, {name}. You are {age} years old.".format(name="Alice", age=30)
print(greeting)

I appreciate the variable naming here because it enhances readability, especially when dealing with many variables, although it is quite verbose.

However, this is great if you have all your variables in a python dictionary. For example:

user = {
    "name": "Alice",
    "age": 30
}
greeting = "Hello, {name}. You are {age} years old.".format(**user)
print(greeting)

Using f-strings

name = "Alice"
age = 30
greeting = f"Hello, {name}. You are {age} years old."
print(greeting)

I prefer f-strings over str.format() with named variables due to their brevity and readability. F-strings also allow operations within them. For example:

a = 1
b = 2

def add(a, b):
    return a+b

text = f"""a = {a}
b = {b}
c = a + b = {add(a, b)}"""

print(text)

Using Template String

from string import Template

t = Template("Hello, $name. You are $age years old.")
greeting = t.substitute(name="Alice", age=30)
print(greeting)

This method might be useful for processing user input, but I find it too verbose and prefer other alternatives.

Other Methods

There are a lot of other ways to do string interpolation in Python. For example, using print with sep="" argument, or by joining a list with list.join(""), or by using + operator.

I am not covering them here, because they are less readable and more verbose.

Multi-Line Strings

All five methods support multi-line strings wrapped within """.

a = 1
b = 2

text = """a = {}
b = {}
c = a + b = {}""".format(a, b, a+b)

print(text)

Conclusion

In general, for readability and brevity, I prefer using f-strings. It also supports inline operations, making it very flexible and easy to use.

But, if I have a dictionary with all the required data and no computation is required, then I would go with str.format() with named variables.