How to Format a String in Python: Interpolation, Concatenation, and More

It’s been awhile since I’ve written one of these “how to” articles, but I’m back at it. This time, I want to talk about string formatting using techniques like interpolation and concatenation. In other words, it’s time to finally learn how to format a string in Python

Problem Introduction

Whether we’re trying to prompt a user or output a nice error message, string formatting can always be challenging. After all, the syntax varies from language to language which can feel like learning a metalanguage. For instance, in languages like Java and C, string formatting relies on understanding concepts like variable arguments and format specifiers:

printf("Hi, %s", Jeremy); # Prints "Hi, Jeremy"

Of course, string formatting gets more complicated as we introduce different data types. For example, numbers have their own set of specifiers: %d, %f, etc. And, we can even specify how the numbers look in terms of padding and truncation.

That said, I don’t you’re here to learn string formatting in C, so how do we accomplish the same thing in Python? In this article, we’ll take a look at several methods—some silly—just to illustrate how many ways there are to solve this problem.

To get started, we’ll need a universal example which contains a few pitfalls like mixing numbers and strings. The following code snippet will serve as our base for the remainder of the article:

name = "Jeremy"
age = 25

Using these variables, we’ll want to construct the following sentence:

print("My name is Jeremy, and I am 25 years old.")

Of course, feel free to swap the name and age with your name and age!

Solutions

As it turns out, there are quite a few ways to format a string. We’ll start from a few direct approaches, then we’ll move into some the for elegant solutions.

Format a String Using Concatenation

If you’re like me, concatenation is something you learned when you first started to code. As a result, concatenation can seem like a quick shortcut to string formatting:

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

Unfortunately, a solution like this won’t work. If you tried to run this code, you’ll get a nasty error that looks something like this:

Traceback (most recent call last):

File “”, line 1, in

“My name is ” + name + “, and I am ” + age + ” years old.”

TypeError: can only concatenate str (not “int”) to str

Hopefully, the TypeError gives you the hint that the interpreter doesn’t like it when we try to concatenate a string with an integer. In other words, we need to cast the age variable to a string:

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

And, that’s it! For small strings, this is probably fine, but it’s not super readable. Also, it’s really easy to forget spaces on either side of the variables we’re concatenating. Luckily, there are other ways to build a string.

Format a String Using Multiple Print Statements

Who needs concatenation when we can just call print a bunch of times?

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

Now, I know what you’re thinking; yes, this only works in Python 3+. Oh, and this is a totally ridiculous solution, but it demonstrates something important: there are a lot of ways to solve the same problem.

In this case, we’ve taken the print function and leveraged one of its default arguments (end) to remove the newline behavior. That way, we could string together some text without concatenation.

Again, this is definitely hard to read, and I wouldn’t even advise it for small strings. That said, it does eliminate a type cast. Unfortunately, it introduces a lot of duplicate code.

Format a String Using the Join Function

Continuing our quest for the most ridiculous way of formatting a string, I bring you the join function. If you’re not familiar with this function, it’s basically a more efficient way to concatenate strings. In addition, it allows us to provide a separator to place between our concatenated strings. Of course, we won’t be needing that:

print(''.join(["My name is ", name, ", and I am ", str(age), " years old"]))

Here, we’ve called the join method on an empty separator string. As an argument, we’ve passed it a list of strings. Naturally, join will combine this list of strings into a single string without any separators.

Oddly enough, I sort of like this solution because it’s surprisingly readable. Unfortunately, there are a few drawbacks. For instance, we have to convert all our variables to strings manually. In addition, this line is already pretty long. Though, I suppose we could break everything out onto its own line.

At any rate, with these three out of the way, we can finally start getting to some more reasonable solutions.

Format a String Using the % Operator

Now, we’re starting to get into the actual string formatting techniques. As it turns out, Python has its own set of formatting tools similar to printf from C:

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

Here, we’ve constructed a new string with %s replaced by name and %d replaced by age.

In addition to knowing the format specifiers, we’ll want to learn the syntax. In particular, our template string is following by the modulo operator. Of course, in this context, we can call it the string formatting or interpolation operator.

Then, we create a tuple of values that we want to place in our string. Be very careful to ensure the order of these values. If they are out of order, the resulting string may be incorrect, or the program may crash altogether.

With this method, we get a much cleaner solution. Of course, there are pitfalls here, but they mostly have to do with how the values are mapped to the string. For example, we have to pay attention to how we order our arguments, and we need to know our format specifiers.

Speaking of format specifiers, what if we want to print out an object directly? Fortunately, we have better solutions ahead.

Format a String Using the Format Function

Instead of using a fancy overloaded operator, we can make our code even more readable by using the format function for strings:

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

Previously, we would have had to use format specifiers to get the behavior we wanted, but now we can just use braces. In other words, we’ve eliminated a problem from the previous solution.

From what I understand, this method leverages the __format__ method for objects, so we can pass just about anything to this method without issue. There goes yet another problem! Of course, if the class doesn’t have __str__ or __repr__ overridden, then the object will not print nicely. That said, I still count that as a win over the previous solution.

As it turns out, we can eliminate our ordering issue from the previous solution as well. All we have to do is provide keyword arguments:

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

In this example, we named the age keyword a and the name keyword n. That way, we could place the keywords inside their respective braces. To further drive home the point, we can even reorder the arguments without issue. Now that’s pretty cool!

Of course, I should warn you that this solution may pose a security threat to your application depending on how you’re using it. If you’re writing your own format strings, there shouldn’t be any issues. However, if your accepting format strings from your users, you might want to be careful.

Format a String Using f-Strings

Another way to perform string interpolation is using Python’s latest f-String feature (Python 3.6+). With this feature, all we need to do is prefix a string with the letter f and insert braces just like before. However, this time, we can insert the name of our variables directly:

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

Now, that is incredibly elegant. No longer do we have to worry about:

• Mapping arguments to format specifiers
• Using format specifiers correctly
• Remembering obscure syntax

Instead, we prepend and f and insert our variables. That’s it! Now, I don’t know if there are any sort security vulnerabilities with this solution, but as far as I can tell, there’s no way to apply the f to an input string.

At any rate, that’s all I have for string formatting solutions. Now, let’s start to compare the performance of these solutions.

Performance

As always, I like to setup on all of our solutions in strings first:

setup = """
name = "Jeremy"
age = 25
"""

concatenation = """
"My name is " + name + ", and I am " + str(age) + " years old."
"""

string_join = """
''.join(["My name is ", name, ", and I am ", str(age), " years old"])
"""

modulus = """
"My name is %s, and I am %d years old." % (name, age)
"""

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

format_named = """
"My name is {n}, and I am {a} years old".format(a=age, n=name)
"""

f_string = """
f"My name is {name}, and I am {age} years old"
"""

For my sanity, I had to remove the print statements. As a result, I wasn’t able to test the print solution. That said, feel free to try your hand at it. I ran into some issues with the output string slowing down the test, and I even tried rerouting stdout to deal with it. It was a nightmare to say the least.

At any rate, it’s just a matter of calling our timeit commands now:

>>> import timeit
>>> min(timeit.repeat(stmt=concatenation, setup=setup, repeat=10))
0.4947876000000022
>>> min(timeit.repeat(stmt=string_join, setup=setup, repeat=10))
0.37328679999995984
>>> min(timeit.repeat(stmt=modulus, setup=setup, repeat=10))
0.29478180000000265
>>> min(timeit.repeat(stmt=format_ordered, setup=setup, repeat=10))
0.40419490000000735
>>> min(timeit.repeat(stmt=format_named, setup=setup, repeat=10))
0.49794210000000305
>>> min(timeit.repeat(stmt=f_string, setup=setup, repeat=10))
0.1918610999999828

As is often the case with these new features in Python, they are incredibly optimized. As a matter of fact, the only solution that even comes close to competing with the f-String solution is the modulus operator solution.

Also, I think it’s worth noting how much slower the format function is when the arguments are named rather than ordered. In fact, it’s about as slow as concatenation which I expected to be horrendous. After all, strings are immutable, so concatenation should be pretty bad.

As always, take these performance metrics with a grain of salt.

A Little Recap

With all that said, here are all of the solutions in one unified location:

name = Jeremy
age = 25

# String formatting using concatenation
print("My name is " + name + ", and I am " + str(age) + " years old.")

# String formatting using multiple prints
print("My name is ", end="")
print(name, end="")
print(", and I am ", end="")
print(age, end="")
print(" years old.")

# String formatting using join
print(''.join(["My name is ", name, ", and I am ", str(age), " years old"]))

# String formatting using modulus operator
print("My name is %s, and I am %d years old." % (name, age))

# String formatting using format function with ordered parameters
print("My name is {}, and I am {} years old".format(name, age))

# String formatting using format function with named parameters
print("My name is {n}, and I am {a} years old".format(a=age, n=name))

# String formatting using f-Strings (Python 3.6+)
print(f"My name is {name}, and I am {age} years old")

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• That Time I Shipped Insecure Code
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