4. Advanced Formatting

In Chapter 2, we have seen the basic string formatting. In this chapter, we will see some more advanced formatting examples upon on that.

4.1. Formatting with placeholders

Let's see the following example first:

Input

species_1 = {'species': 'Tub', 'age': 5}
sentence = 'My name is ' + species_1['species'] + ' and I am ' + str(species_1['age']) + ' years old.'
print(sentence)

Output

My name is Tub and I am 5 years old.

We can see there are a lot of blank space we have to manually put in the beginning and the end of each string. We also have to cast the integer/number, species_1['age'], into strings by using str().

Of course, there is a better way to do this:

Input

sentence = 'My name is {} and I am {} years old.'.format(species_1['species'], species_1['age'])
print(sentence)

Output

My name is Tub and I am 5 years old.

We can see that we don't have to cast the integer/number into strings anymore.

We can also use the index of the placeholder to specify the order of the arguments:

Input

sentence = 'My name is {0} and I am {1} years old.'.format(species_1['species'], species_1['age'])
print(sentence)

Output

My name is Tub and I am 5 years old.

This is another example using the index of the placeholder with repeated arguments:

Input

tag = 'p'
text = 'This is a paragraph'
sentence = '<{0}>{1}</{0}>'.format(tag, text)
print(sentence)

Output

<p>This is a paragraph</p>

This is very useful when we have a placeholder that you want to reuse. Let's see another example:

Input

sentence = 'My name is {0} and I am {1} years old. My friend Barkalot is also {1} years old.'.format(
    species_1['species'], species_1['age'])
print(sentence)

Output

My name is Tub and I am 5 years old. My friend Barkalot is also 5 years old.

There is another way to do this by using the key in the placeholder instead of the index:

Input

sentence = 'My name is {0[species]} and I am {1[age]} years old.'.format(species_1, species_1)
print(sentence)

Output

My name is Tub and I am 5 years old.

Here, we call the value of the key species in the dictionary species_1 by using {0[species]}.

But this is redundant because we have to repeat the same argument twice.

We can do the following instead:

Input

sentence = 'My name is {0[species]} and I am {0[age]} years old.'.format(species_1)
print(sentence)

Output

My name is Tub and I am 5 years old.

We can also use the index of a list in the placeholder:

Input

species = ['Tub', 'Furrytail', 'Cat', 'Barkalot']
sentence = 'I am {0[1]} and my friend is {0[3]}.'.format(species)
print(sentence)

Output

I am Furrytail and my friend is Barkalot.

We can also access the attributes of an object in the same way. For example, we define a class called Species that represents a species with a species and an age. The class has an __init__ method that initializes the instance variables species and age. We then create an instance of the Species class called species_new with the name 'Jerry' and the age 88.

You then create an instance of the Species class called species_new with the name 'Jerry' and the age 88.

Input

class Species:

    def __init__(self, species, age):
        self.species = species
        self.age = age

species_new = Species('Jerry', 88)

Now we can access the attributes of the object species_new by using the index of the placeholder:

Input

sentence = 'My name is {0.species} and I am {0.age} years old.'.format(species_new)
print(sentence)

Output

My name is Jerry and I am 88 years old.

The string contains placeholders {0.species} and {0.age} which will be replaced by the species and age attributes of the species_new object. The 0 in the placeholders refers to the first argument passed to the .format() method, which is species_new.

This time, we are using keyword arguments to pass the values that will replace the placeholders in the string. The string contains placeholders {species} and {age}, which will be replaced by the values provided as keyword arguments in the .format() method.

Input

sentence = 'My name is {species} and I am {age} years old.'.format(species='Jerry', age=88)
print(sentence)

Output

My name is Jerry and I am 88 years old.

4.2. `**` in `.format()

In the format() method, the ** operator can be used to unpack a dictionary that contains the keyword arguments for the placeholders in the string. This can be a convenient way to format a string using a dictionary that has keys matching the placeholders in the string.

Here's an example:

Input

pet = {'species': 'Tub', 'age': 5}
sentence = 'My name is {species} and I am {age} years old.'.format(**pet)
print(sentence)

Output

My name is Tub and I am 5 years old.

In this example, we have a dictionary pet containing the keys species and age. We then use the ** operator to unpack the dictionary when calling the .format() method on the string. The values from the dictionary are used to replace the placeholders in the string, resulting in the sentence "My name is Tub and I am 5 years old."

This method is useful when we print out the dictionaries, which is more readable.

why use **species instead of species?

When you use ** before a dictionary in a function call, like in the .format() method, it is known as dictionary unpacking. It allows you to pass the key-value pairs in the dictionary as named (keyword) arguments to the function.

When a dictionary is passed as a keyword argument in this way, the keys in the dictionary are treated as the parameter names and the corresponding values are passed as the parameter values.

So, in this case, the keys in the pet dictionary ('species' and 'age') are treated as parameter names in the sentence string, and their corresponding values ('Tub' and 5) are passed as parameter values.

If you were to pass the pet dictionary without the double asterisks, like so:

Input

sentence = 'My name is {species} and I am {age} years old.'.format(species)
print(sentence)

Output

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'species'

The .format() method would be looking for a single argument that is a dictionary, rather than separate keyword arguments, and would raise a TypeError.

4.3. Formatting Numbers

In this block, we are using the .format() method to insert values into a string. The {} serves as a placeholder for the value that will be inserted. The output will be a series of strings with the respective values from the loop.

Input

for i in range(1, 6):
    sentence = 'The value is {}'.format(i)
    print(sentence)

Output

Output:
The value is 1
The value is 2
The value is 3
The value is 4
The value is 5

Padding with zeros (width 2): In this block, we're using the :02 format specifier within the placeholder to pad the value with zeros, ensuring a minimum width of 2 characters. This is useful for maintaining consistent formatting when dealing with numbers of varying lengths.

Input

for i in range(1, 6):
    sentence = 'The value is {:02}'.format(i)
    print(sentence)

Output

Output:
The value is 01
The value is 02
The value is 03
The value is 04
The value is 05

Padding with zeros (width 3): Similarly, we can use the :03 format specifier to pad the value with zeros, ensuring a minimum width of 3 characters. This results in a more extensive padding for the smaller numbers, keeping the output format consistent.

Input

for i in range(1, 6):
    sentence = 'The value is {:03}'.format(i)
    print(sentence)

Output

Output:
The value is 001
The value is 002
The value is 003
The value is 004
The value is 005

By using :.2f within the placeholder, we can format a floating-point number to display two decimal places.

Input

# We can also use the `:.2f` with two decimal places
e = 2.71828
sentence = 'e is equal to {:.2f}'.format(e)
print(sentence)

Output

e is equal to 2.72

By using :, within the placeholder, we can format a number with a thousands separator, ,.

Input

sentence = '1 KM is equal to {:,.2f} meters'.format(1000)
print(sentence)

Output

1 KM is equal to 1,000.00 meters

4.4. Formatting Date and Time with `datetime`

When formatting date and time, we can refer to the strftime and strptime behavior for the format codes.

In the following code, we are using Python's datetime module to create a datetime object representing a specific date and time. We create a datetime object for April 1, 2023, 10:10:30 AM, and print it.

Input

import datetime
today_date = datetime.datetime(2023, 4, 1, 10, 10, 30)
print(today_date)

Output

2023-04-01 10:10:30

We can use the strftime method to format the date in a more human-readable format. The strftime method allows you to create custom date and time formats by using format codes. We can use the strftime method to format the date.

But we have to import the datetime module first.

Input

import datetime
today_date = datetime.datetime(2023, 4, 1, 10, 10, 30)
today_date = '{:%B %d, %Y}'.format(today_date)
print(today_date)

Output

April 01, 2023

Here, we use the format codes %B, %d, and %Y to display the full month name, the day of the month with a leading zero, and the year with the century, respectively. The resulting formatted date is passed to the format function and printed.

We can also use the strptime method to parse the string into a date:

Input

import datetime
date_str = 'April 01, 2023'
today_date = datetime.datetime.strptime(date_str, '%B %d, %Y')
print(today_date)

Output

2023-04-01 00:00:00

n this example, we have the date string 'April 01, 2023'. The format codes used to parse this string are%B,%d, and%Y`, which represent the full month name, the day of the month with a leading zero, and the year with the century, respectively. The strptime method reads the date string and creates a datetime object with the provided date information.

Input

import datetime
today_date = datetime.datetime(2023, 4, 1, 10, 10, 30)
sentence = 'Today is {0:%B %d, %Y} on {0:%A}, and {0:%Y} has passed {0:%j} days'.format(today_date)
print(sentence)

Output

Today is April 01, 2023 on Thursday, and 2023 has passed 091 days

We can also use the timetuple method to remove the 0 in the beginning of 091:

Input

sentence = 'Today is {0:%B %d, %Y} on {0:%A}, and {0:%Y} has passed {1:d} days'.format(today_date, today_date.timetuple().tm_yday)
print(sentence)

Output

Today is April 01, 2023 on Thursday, and 2023 has passed 91 days

4. Advanced Formatting

4.1. Formatting with placeholders

4.2. ** in `.format()

4.3. Formatting Numbers

4.4. Formatting Date and Time with datetime

4.2. `**` in `.format()

4.4. Formatting Date and Time with `datetime`