Python String Concatenation

String concatenation in Python is joining two or more strings into a single one. As a result, we create a new string that contains all the characters from the original string in the correct order. For example, if we have two strings, Hello and world!, concatenating them will produce the new string Hello world!.

In Python, as in many other programming languages, there are different ways to concatenate strings. Let's look at all of them.

The + operator

Let's take a classic example:

string1 = "Hello"
string2 = "world!"
string3 = string1 + ", " + string2
print(string3)

Hello, world!

Here we have given string1 the value Hello and string2 the value world!, then added them together by adding a comma with a space in the middle and printed the new string.

Here are a couple more examples of concatenation using the + operator. Here is the concatenation of three strings:

string1 = "I"
string2 = "love"
string3 = "Python"
result = string1 + " " + string2 + " " + string3
print(result)

I love Python

As you can see, there are no differences. And this example:

first_name = "Monty"
last_name = "Python"
result = "My name is " + first_name + " " " + last_name
print(result)

My name is Monty Python

Note that in both examples, we have added spaces between words to get the correct output from a printing perspective. It is recommended to do this in other cases as well.

The join() method

In the examples above, we used the + operator to join two or more strings. But string concatenation in Python 3 can also be done using the join() method. Here is our first example written in this way:

strings = ["Hello", "world!"]
string3 = ", ".join(strings)
print(string3)

In the output, we will see the same Hello, world!.

Here are two other examples rewritten using the join() method:

strings = ["I", "love", "Python"]
result = " ".join(strings)
print(result)

I love Python

first_name = "Monty"
last_name = "Python"
strings = ["My name is", first_name, last_name]
result = " ".join(strings)
print(result)

My name is Monty Python

In both examples, we created a list of strings and then used the join() method to join them into a single string. In the first example, we only added a space to separate the words, while in the second example we also involved variables in the list.

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String and number concatenation in Python

If we try to concatenate a string and a number, we get an error:

age = 28
message = "I am " + age + " years old."
print(message)

Traceback (most recent call last):
  File "C:/Python/Python38/concat1.py", line 2, in <module>
    message = "I am " + age + " years old."
TypeError: can only concatenate str (not "int") to str

This happens when we use an incorrect syntax. In this case, the code will cause a TypeError because we are trying to concatenate a string with a number without converting the number to a string as well.

Concatenation via str()

To fix this error, we can use str() to convert a number to a string. Here is an example of proper string and number concatenation:

age = 28
message = "I am " + str(age) + " years old."
print(message)

I am 28 years old.

We converted the number enclosed in the age variable into a string using str(), and the interpreter gave us the desired result.

Concatenation using format()

The same thing can be done using the format() method:

age = 28
message = "I am {} years old.".format(age)
print(message)

I am 28 years old.

This method works the same way as str(), only the syntax differs.

Concatenation via f-string

Another way to get the same result is to use a new feature of version 3, f-string:

age = 28
message = f"I am {age} years old."
print(message)

I am 28 years old.

Python's f-string (formatted string literals) works like the str.format() method but provides a more convenient and concise way to put values into a string. With f-string, we can insert variable values directly inside a string by using curly braces {} and specifying the variable name inside those braces. Here are some more examples of string and number concatenation using f-string:

name = "Monty"
age = 35
print(f"My name is {name} and I am {age} years old.")

My name is Monty and I am 35 years old.

salary = 100000
tax_rate = 0.25
taxes = salary * tax_rate
print(f"My salary is {salary} and my taxes are {taxes}.")

My salary is 100000 and my taxes are 25000.0.

As you can see, we can insert variable values inside a string using curly braces, and Python automatically substitutes variable values in these places when the program is executed. Note that f-string also supports various output formats that can be specified inside curly braces. For example, you can specify the number of decimal places for floating point numbers. This makes f-string an even more powerful formatting tool.

Conclusion

With the concatenation, we have once again seen how flexible Python is, because you can join strings using a variety of methods. Even concatenating a string to a number by converting a numeric value to a string value can be done in at least three ways. Just choose your favorite.