String Formatting

July 2021

Table of Contents

Summary

A brief survey of Python’s string formatting methods. There are mainly three ways to format strings in Python. C-style formatting with % is the oldest, then came along str.format() with Python3, and finally f-strings with Python 3.6. F-string interpolation is arguably the most effective way to format strings in Python, but it’s good to know all the different possibilities.

1. C-style formatting (using the % operator)

This capability is modeled after C. It’s the original string-formatting method in Python. It works, but has a few drawbacks.

% operator with a tuple

tple = ("1st", "3rd", "2nd", "1st")
print("Return the %s, the %s, the %s, and finally the %s again" % tple)
# Return the 1st, the 3rd, the 2nd, and finally the 1st again

Disadvantages:

Both sides must remain in sync at all times. ie. order is “hardcoded” and has to be prioritized in case of edits.
Any one value has to be repeated (in the tuple) as many times as it is needed in the formatted string, creating unnecessary noise.

% operator with a dictionary

us_capitals = {"OR": "Salem", "WA": "Olympia", "ID": "Boise"}
formatted = (
    "Oregon's capital is %(OR)s, "
    "Washington's capital is %(WA)s, "
    "Idaho's capital is %(ID)s"
)
print(formatted % us_capitals)
# Oregon's capital is Salem, Washington's capital is Olympia, Idaho's capital is Boise

2. str.format()

This formatting method introduced curly braces, {}, as placeholders within strings. Same as with C-style formatting, the string to format and the contents to format it with live in separate places. Specifically, it follows this structure: "...{}...{}...".format(element1, element2); alternatively, the curly braces can hold the index of the element, to refer to it positionally, like so: "...{1}...{0}...".format(element1, element2) (where index 0 retrieves element1 and index 1 retrieves element2). In addition, this update included a powerful Format Specification Mini-Language with features to manipulate string/digits in terms of precision, alignment, width, capitalization, etc.

Basic demo

"{0}, {1}, {2}".format("a", "b", "c")
# 'a, b, c'
"{}, {}, {}".format("a", "b", "c")  # 3.1+ only
# 'a, b, c'
"{2}, {1}, {0}".format("a", "b", "c")
# 'c, b, a'
"{2}, {1}, {0}".format(*"abc")  # unpacking argument sequence
# 'c, b, a'
"{0}+{1}+{0}".format("abra", "cad")  # arguments' indices can be repeated
# 'abra+cad+abra'

Unpacking a dictionary’s values with str.format()

us_capitals = {"OR": "Salem", "WA": "Olympia", "ID": "Boise"}
formatted = (
    "Oregon's capital is {OR},"
    "Washington's capital is {WA},"
    "Idaho's capital is {ID}"
)
print(formatted.format(**us_capitals))
# Oregon's capital is Salem,Washington's capital is Olympia,Idaho's capital is Boise

Vis-a-vis C-style formatting, this method is more straightforward, having gotten rid of the need to add the % and the s. But it’s brittle in that it still depends on the ordering of the tuple’s elements (if used with a tuple). Using it with a dictionary makes it more robust.

3. f-string Interpolation (Python 3.6+)

With f-string interpolation variables or expressions are placed direcly inside curly braces (together with any extra formatting), which makes the whole formatted string self-contained. Thus making for a concise expression of somewhat terse elegance. The structure it follows is: f"...{<var>}...{<expr>}...".

Variables within {} placeholders

color = "blue"
weight = "2ton"
discount = 9.99
decimals = 2
formatted = f"The car is {color}, weighs {weight} and is for sale \
with a {discount:.{decimals}f}% discount"
print(formatted)
# The car is blue, weighs 2ton and is for sale with a 9.99% discount

Expressions within {} placeholders

# capitalize the brand and double the stock found in the garage
garage = [("tesla", 2), ("volvo", 1), ("bmw", 2)]

for i, (brand, qty) in enumerate(garage):
    row = f"row id:{i}, car maker:{brand.capitalize()}, stock:{2*qty}"
    print(row)

# row id:0, car maker:Tesla, stock:4
# row id:1, car maker:Volvo, stock:2
# row id:2, car maker:Bmw, stock:4

Extracting values from a dictionary

us_capitals = {"OR": "Salem", "WA": "Olympia", "ID": "Boise"}
formatted = (
    f"Oregon's capital is {us_capitals['OR']}, "
    f"Washington's capital is {us_capitals['WA']}, "
    f"Idaho's capital is {us_capitals['ID']} "
)
print(formatted)
# Oregon's capital is Salem, Washington's capital is Olympia, Idaho's capital is Boise

Rafael Marino