Formatting strings
We have also used f-strings (formatted string literals).
f-strings were introduced in Python 3.6 to make string formatting easier and more readable. Before f-strings, the two main methods for formatting strings are:
name = "Josiah"
count = 10
possession = "cars"
# Python 2 "old-style" strings
print("%s has %d %s." % (name, count, possession))
# Python 3 str.format()
print("{} has {} {}.".format(name, count, possession))
print("{name} has {count} {possession}.".format(name=name,
count=count,
possession=posession
))
(And no, I do not own any cars!)
We will not cover these old methods since they are no longer recommended (read this article if you are interested). It is enough to know f-strings are designed to fix the weaknesses of older methods.
In fact, I did not even know about f-strings until I started preparing for this course, and will be using it from now on!
print(f"{name} has {count} {possession}")
Multiple lines can be handled with paranthesis:
name = "Josiah"
age = "18"
occupation = "Senior Teaching Fellow"
profile = (
f"Name: {name}\n"
f"Age: {age}\n"
f"Occupation: {occupation}"
)
print(profile)
You can have any expression in f-strings. Python will eval() whatever is between the curly braces at runtime (i.e. parse the string and run the string as code).
print(f"Multiplying count by 5 gives you {count * 5}")
print(f"Is count equal to 10? {count == 10}")
You can also access indices of dictionaries or lists directly in f-strings. Be careful with the single and double quotes!
my_dict = {"key1": "value1"}
print(f"{my_dict['key1']}")
In fact, you can include any object (implement the __str__ or __repr__ magic methods for your custom classes if you want to output something more informative)
class MyClass:
def __str__(self):
return "an instance of MyClass"
print(f"This is {MyClass()}.")
You can also call functions directly inside f-strings (but assigning them to variables might make them much more readable).
numbers = [6,3,9,2,4,5]
print(f"Count: {len(numbers)}; Sum: {sum(numbers)}; Mean: {sum(numbers)/len(numbers)}")
You can format floating point values.
pi = 3.14159265359
print(f"PI with three significant digits: {pi:.3}")
print(f"PI with three decimal points: {pi:.3f}")
print(f"PI with three significant digits, 9-character width is {pi:9.3}")
print(f"PI with three significant digits, 9-character width padded with 0 is {pi:09.3}")
And you can left/centre/right justify strings (giving a fixed length).
title = "Title"
print(f"| {title : <20} | {title : ^20} | {title : >20} |")
For Python>=3.8 only! f-string debugging: you can create self-documenting expressions with the = operator. The example will probably explain this better.
numbers = [6,3,9,2,4,5]
print(f"{sum(numbers) = }")