String formatting is an essential part of Python programming for web development, data analysis, machine learning, and more.

In earlier posts, we discussed set up Python development environment, including installing the Windows Terminal, adding Anaconda Prompt on the Windows terminal, adding it on the Windows right-click menu, adding Anaconda PowerShell Prompt on the Windows terminal, or adding it on the Windows right-click menu. Besides, more recently, I introduced an Easy Way to Add Anaconda PowerShell Prompt on the Windows Terminal.

In this post, we will discuss on Python string formatting methods, and 5 essential string formatting methods in Python will be displayed with concrete examples.

1. Comma separated method

In most simple cases, we can use comma separated formatting method, like print('string', variable). For example:

(1) One variable

In [3]:

score = 95

print('The average score of Group A:', score)

The result is as follows:

The average score of Group A: 95

(2) More than one variable

In [2]:

group = 'Group A'
score = 95

print('The average score of', group, ':', score)

The output is:

The average score of Group A : 95

(3) A list

In [4]:

score_list = [95,96,86,55]
print("The average scores of Group A, Group B, Group C and Group D are", score_list)

The result is as follows:

The average scores of Group A, Group B, Group C and Group D are [95, 96, 86, 55]

2. Concatenation method

Similarly, we can use ‘+’ sign to concatenate string and variables, like print('string+ '' + variable). Let’s see the same example as above. This method can only be used to concatenate string variables. But we can change a numeric variable into a string, or use the comma method.

(1) One variable

In [9]:

score = 95

print('The average score of Group A: ' + str(score))

The result is as follows:

The average score of Group A: 95

(2) More than one variable

In [11]:

group = 'Group A'
score = 95

print('The average score of' + group + ': ' + str(score))

The result is as follows:

The average score of Group A: 95

In [12]:

# or 

print('The average score of' + group + ':', score)

It produces the same result:

The average score of Group A: 95

(3) A list

In [14]:

score_list = [95,96,86,55]
print("The average scores of Group A, Group B, Group C and Group D are " + str(score_list))

The result is as follows:

The average scores of Group A, Group B, Group C and Group D are [95, 96, 86, 55]

The cons of these two methods are convenient for simple cases, one or two variables. So some advanced methods are designed for more complex cases. Let’s see other 3 essential string formatting methods in Python as follows.

3. % formatting

This is a comparatively older method of string formatting in Python, which uses the % operator. The %s marker inserts a string, the %d marker inserts an integer, and the %f marker inserts a float. Let’s see some examples.

(1) One variable

In [15]:

name = 'Mike'
print('Hello, %s!'%name)

It generates the following result:

Hello, Mike!

The variable name is a string, thus it uses %s.

(2) More than one variable

In [16]:

name = 'Jack'
age = 20

print('%s is %d years old.' %(name,age))

The result is as follows:

Jack is 20 years old.

The variable name is a string and age is an integer, thus we use %s and %d, respectively.

(3) A list

In [17]:

alist = [5,1,8]
print("A list: %s" %alist)

The result is as follows:

A list: [5, 1, 8]

(4) Format the number value

We usually use Python to make calculations, where the floating numbers are usual. Let’s see how to format float decimal place with certain decimal places, such as 0.2, 0.22, …

In [18]:

x = 1
y = 3
z = x/y

print('The result of %d divided by %d is %f.'%(x,y,z))
print('The result of %d divided by %d with one decimal '\
    'place is %.1f.'%(x,y,z))
print('The result of %d divived by %d with '\
    'two decimal places is %.2f.'%(x,y,z))
print('The result of %d divided by %d with '\
    'three decimal places is %.3f.'%(x,y,z))

The results are as follows:

The result of 1 divided by 3 is 0.333333.
The result of 1 divided by 3 with one decimal place is 0.3.
The result of 1 divived by 3 with two decimal places is 0.33.
The result of 1 divided by 3 with three decimal places is 0.333.

This method is confused sometime with different marks for different data types. Thus, more convenient methods are created in Python, such as curly brace string formatting and f-string method.

4. Curly brace string formatting

The advantage of this method is that you can insert more than one values using curly brace, and the values can be numbers and other Python objects.

(1) Insert a string and number

In [1]:

name = 'Jack'
age = 20

print ('{} is {} years old.'.format(name, age))

The result is as follows:

Jack is 20 years old.

(2) Insert a complex data type

We can easily insert a complex data types, such as list, tuple, etc.

In [5]:

alist = [5,1,8]
print("A list: {}.".format(alist))

The result is as follows:

A list: [5, 1, 8].

(3) Format the number value

Let’s see the same example to display how to format float decimal places.

In [19]:

x = 1
y = 3
z = x/y

print('The result of {} dived by {} is {}.'.format(x,y,z))
print('The result of {} dived by {} with one decimal'\
    'place is {:.1f}.'.format(x,y,z))
print('The result of {} dived by {} with'\
    'two decimal places is {:.2f}.'.format(x,y,z))
print('The result of {} dived by {} with'\
    'three decimal places is {:.3f}.'.format(x,y,z))

The results are as follows:

The result of 1 dived by 3 is 0.3333333333333333.
The result of 1 dived by 3 with one decimal place is 0.3.
The result of 1 dived by 3 with two decimal places is 0.33.
The result of 1 dived by 3 with three decimal places is 0.333.

5. f-string method

This is a comparatively new method, which is only after Python >= version 3.6. An f prefix at the beginning of the string tells Python to insert any currently valid variables into the string. It is the most convenient and practical string formatting method in Python at the moment. We cannot cover all the functions of this method, so here we only take some examples to compare with the above methods.

(1) One variable

In [6]:

name = 'Jack'

print(f'Hello, {name}.')

The result is as follows:

Hello, Jack.

(2) More than one variable

In [74]:

name = 'Jack'
age = 20

print(f'{name} is {age} years old.')

The result is as follows:

Jack is 20 years old.

(3) f-string list

In [9]:

alist = [5,1,8]

print(f"A list: {alist}")

The result is as follows:

A list: [5, 1, 8]

The result is as follows:

(4) Formatting floats

In [4]:

x = 1
y = 3
z = x/y

print(f'{x} is dived by {y} is {z}.')
print(f'{x} is dived by {y} is {z:.1f}.')
print(f'{x} is dived by {y} is {z:.2f}.')
print(f'{x} is dived by {y} is {z:.3f}.')
print(f'{x} is dived by {y} is {z:.4f}.')

The result is as follows:

1 is dived by 3 is 0.3333333333333333.
1 is dived by 3 is 0.3.
1 is dived by 3 is 0.33.
1 is dived by 3 is 0.333.
1 is dived by 3 is 0.3333.

(5) f-string Dictionaries

Let’s see a dictionary example as follows.

In [5]:

fruit = {
  'name': 'Apple', 
  'price': '3.0'
}

print(f"{fruit['name']} is ${fruit['price']}")

The result is as follows:

Apple is $3.0

(6) f-string expression

It can also insert a calculation express. For example:

In [12]:

apple_amount = 5  # kg
cost = 3.0 # Dollar per kg

print(f'Total cost of the apple is ${apple_amount * cost}.')

The result is as follows:

Total cost of the apple is $15.0.

(7) multiline f-string

Another convenient thing of this method is that we can insert a multi-line string.

In [14]:

name = 'Jack Smith'
age = 25
occupation = 'Professor'

file = (
    f'Name: {name}\n'
    f'Age: {age}\n'
    f'Occupation: {occupation}'
)
print(file)

The result is as follows:

Name: Jack Smith
Age: 25
Occupation: Professor

(8) f-string calling function

Besides, we can also call a function directly.

In [15]:

def additor(x, y):

    return x + y

a = 5
b = 7

print(f'Sum of {a} and {b} is {additor(a, b)}')

The result is as follows:

Sum of 5 and 7 is 12

(9) f-string objects

Besides the function, it can also be used in the class objects, but the objects must have either str() or repr() magic functions defined in the class.

In [6]:

class User:
    def __init__(self, name, occupation):
        self.name = name
        self.occupation = occupation

    def __repr__(self):
        return f"{self.name} is a {self.occupation}"

u = User('Jack Lee', 'Professor')

print(f'{u}')

The result is as follows:

Jack Lee is a Professor

(10) f-string format width

The value may be filled with spaces or other characters if the value is shorter than the specified width
In the following example, it prints three columns, and each column has a predefined width. The first column uses 0 to fill shorter values.

In [7]:

for x in range(0, 10):
    print(f'{x:02} {x*x:3} {x*x*x:4}')

The result is as follows:

00   0    0
01   1    1
02   4    8
03   9   27
04  16   64
05  25  125
06  36  216
07  49  343
08  64  512
09  81  729

Let’s see another example, where we align strings to the right with specified width of 5 characters. The first four lines use spaces to fill the shorter values.

In [9]:

s1 = '1'
s2 = '12'
s3 = '123'
s4 = '1234'
s4 = '12345'

print(f'{s1:>5}')
print(f'{s2:>5}')
print(f'{s3:>5}')
print(f'{s4:>5}')

The result is as follows:

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5 Essential String Formatting Methods in Python