String Formatting

Learning Objectives:

At the end of this chapter the students will be able to understand:

1. Formatting using percentage symbol

Python
>>> name = "Eric"
>>> age = 74
>>> "Hello, %s. You are %s." % (name, age)
'Hello Eric. You are 74.'

2. str.format()

With str.format(), the replacement fields are marked by curly braces:

Python
>>> "Hello, {}. You are {}.".format(name, age)
'Hello, Eric. You are 74.'

You can reference variables in any order by referencing their index:

Python
>>>"Hello, {1}. You are {0}.".format(age, name)
'Hello, Eric. You are 74.'

But if you insert the variable names, you get the added perk of being able to pass objects and then reference parameters and methods in between the braces:

Python
>>> person = {'name': 'Eric', 'age': 74}
>>> "Hello, {name}. You are {age}.".format(name=person['name'], age=person['age'])
'Hello, Eric. You are 74.'

You can also use ** to do this neat trick with dictionaries:

Python
>>> person = {'name': 'Eric', 'age': 74}
>>> "Hello, {name}. You are {age}.".format(**person)
'Hello, Eric. You are 74

3. f-Strings

The syntax is similar to the one you used with str.format() but less verbose. Look at how easily readable this is:

Python
>>> name = "Eric"
>>> age = 74
>>> f"Hello, {name}. You are {age}."
'Hello, Eric. You are 74.'

It would also be valid to use a capital letter F:

Python
>>> F"Hello, {name}. You are {age}."
'Hello, Eric. You are 74.'

You could also call functions. Here’s an example:

Python
>>> def to_lowercase(input):
...     return input.lower()
>>> name = "Eric Idle"
>>> f"{to_lowercase(name)} is funny."
'eric idle is funny.'

Multiline f-Strings

You can have multiline strings:

Python
>>> name = "Eric"
>>> profession = "comedian"
>>> affiliation = "Monty Python"
>>> message = (
...     f"Hi {name}. "
...     f"You are a {profession}. "
...     f"You were in {affiliation}."
... )
>>> message
'Hi Eric. You are a comedian. You were in Monty Python.'

Speed comparison

The f in f-strings may as well stand for “fast.”

f-strings are faster than both %-formatting and str.format(). As you already saw, f-strings are expressions evaluated at runtime rather than constant values.
Here’s a speed comparison:
Python
>>> import timeit
>>> timeit.timeit("""name = "Eric"
... age = 74
... '%s is %s.' % (name, age)""", number = 10000)
0.003324444866599663
Python
>>> timeit.timeit("""name = "Eric"
... age = 74
... '{} is {}.'.format(name, age)""", number = 10000)
0.004242089427570761
Python
>>> timeit.timeit("""name = "Eric"
... age = 74
... f'{name} is {age}.'""", number = 10000)
0.0024820892040722242

f’string with Dictionaries

If you are going to use single quotation marks for the keys of the dictionary, then remember to make sure you’re using double quotation marks for the f-strings containing the keys.

This will work:
Python
>>> comedian = {'name': 'Eric Idle', 'age': 74}
>>> f"The comedian is {comedian['name']}, aged {comedian['age']}."
The comedian is Eric Idle, aged 74.

Formatting Types

Type Description
:< Left aligns the result (within the available space)
:> Right aligns the result (within the available space)
:^ Center aligns the result (within the available space)
:= Places the sign to the left most position
:+ Use a plus sign to indicate if the result is positive or negative
:- Use a minus sign for negative values only
: Use a space to insert an extra space before positive numbers (and a minus sign befor negative numbers)
: Use a comma as a thousand separator
:_ Use a underscore as a thousand separator
:b Binary format
:c Converts the value into the corresponding unicode character
:d Decimal format
:e Scientific format, with a lower case e
:E Scientific format, with an upper case E
:f Fix point number format
:F Fix point number format, in uppercase format (show inf and nan as INF and NAN)
:g General format
:G General format (using a upper case E for scientific notations)
:o Octal format
:x Hex format, lower case
:X Hex format, upper case
:n Number format
:% Percentage format

:<

>>> txt = "We have {:<8} chickens."
>>> print(txt.format(49))

Output:

>>> We have 49       chickens.

:+

>>> txt = "The temperature is between {:+} and {:+} degrees celsius."

>>> print(txt.format(-3, 7))

Output:

>>> The temperature is between -3 and +7 degrees celsius.

:o

>>> txt = "The octal version of {0} is {0:o}"

>>> print(txt.format(10))

Output:

>>> The octal version of 10 is 12

:f

>>> txt = "The price is {:.2f} dollars."
>>> print(txt.format(45))

Output:

>>> The price is 45.00 dollars.