String Interpolation and Concatenation¶

We learned what the string data type was from our third chapter. A quick recap: Strings are used to represent any textual data.

We also saw how adding two inputs resulted in a different output than we expected.

num1 = input("Enter the first number: ")
num2 = input("Enter the second number: ")
result = num1 + num2

print("The sum of", num1, "and", num2, "is", result)

After running the program:

Enter the first number: 23
Enter the second number: 12
The sum of 23 and 12 is 2312

We briefly touched on the fact that this was caused by string concatenation. In this chapter, we will look at what concatenation is, and at a more powerful way of combining text and variables called string interpolation.

Why do we need this?¶

Most programs need to build sentences out of a mix of fixed text and changing data. For example, greeting a user by name, or showing a player's score. So far, the only way we've done this is by passing multiple values to print:

name = "Alan"
age = 34

print("My name is", name, "and I am", age, "years old.")

This works, but it only works with print. What if we want to, for example, store the full sentence in a variable instead? This is where concatenation and interpolation come in.

String Concatenation¶

Concatenation simply means joining two or more strings together, one after the other. In Python, we can do this with the + operator.

first_name = "Alan"
last_name = "Turing"

full_name = first_name + " " + last_name
print(full_name)

Alan Turing

Notice that we added " " (a string containing a single space) in the middle. Concatenation joins strings exactly as they are, so Python will not add any spaces for us. Without it, we would get AlanTuring as the output.

Concatenating with other data types¶

Concatenation only works between strings. This means that if we try to join a string with a number, Python will raise an error:

name = "Alan"
age = 34

print("My name is " + name + " and I am " + age + " years old.")

TypeError: can only concatenate str (not "int") to str

Python doesn't know whether "I am " + 34 should mean "add" or "join", so instead of guessing, it stops with an error. To fix it, we need to convert the number to a string first. Remember type casting from the previous chapter? We can use the str function for this:

name = "Alan"
age = 34

print("My name is " + name + " and I am " + str(age) + " years old.")

My name is Alan and I am 34 years old.

String Interpolation¶

Interpolation is the process of inserting values directly into a string. Instead of breaking the string apart and gluing the pieces back together, we write the whole sentence at once and mark the places where the values should go. Python gives us two ways of doing this.

f-strings¶

An f-string (short for formatted string literal) is a string with an f placed right before the opening quote. Inside it, anything wrapped in curly braces {} is replaced with its value.

name = "Alan"
age = 34

print(f"My name is {name} and I am {age} years old.")

My name is Alan and I am 34 years old.

That's the same output as before, but much easier to read and write! Also notice that we didn't need to use str(age). This is because f-strings convert the values for us automatically.

Expressions inside f-strings¶

You can put any valid Python expression (not just variable names) inside the curly braces, and Python will work out the result:

price = 20
quantity = 3

print(f"Total cost: {price * quantity}")

Total cost: 60

Formatting values¶

You can also control how a value is displayed by adding a colon : followed by a format specification inside the braces. One of the most common uses of this is limiting the number of decimal places of a float:

pi = 3.14159265

print(f"Pi is roughly {pi:.2f}")

Pi is roughly 3.14

Here, .2f means "show this as a float with 2 digits after the decimal point".

The str.format() method¶

Before f-strings were introduced in Python 3.6, the most common way to interpolate was the format method of strings. You will still see this in a lot of older code, so it is worth knowing.

Instead of placing the variables inside the string, you leave empty curly braces {} as placeholders, and pass the values to .format() afterwards. The values are filled in order like so:

name = "Alan"
age = 34

print("My name is {} and I am {} years old.".format(name, age))

My name is Alan and I am 34 years old.

You can also number the placeholders to control which value goes where, or give them names:

# Using positions
print("{1} is {0} years old.".format(age, name))

# Using names
print("{n} is {a} years old.".format(n=name, a=age))

It's output:

Alan is 34 years old.
Alan is 34 years old.

Which one should I use?¶

All three approaches produce the same kind of result: a new string. Here's a quick comparison using the same example.

name = "Alan"
age = 34

print("My name is " + name + " and I am " + str(age) + " years old.") # concatenation
print("My name is {} and I am {} years old.".format(name, age))       # str.format()
print(f"My name is {name} and I am {age} years old.")                 # f-string

Here are some general rules for their usage.

• Use f-string interpolation for most situations. They are the shortest, the most readable, and the most common way to write modern Python.
• Use concatenation (+) for simple cases where you want to join two strings only.
• str.format() when you are reading older code.