String Formatting

format(), the older % style, and how to line text up in columns.

Why this chapter exists

f-strings are what you should write. You still need to read the two older styles, because they are everywhere in existing code, and one of them is still the right tool when the template is not a literal.

str.format()

Placeholders are empty braces filled from the arguments:

Python

template = "{} is {}"
print(template.format("Ada", 36))

Output

Ada is 36

The advantage over an f-string: the template can be a variable, decided at runtime or loaded from a file. An f-string is interpolated where it is written, so it cannot do that.

Placeholders can be numbered or named, which makes long templates readable and lets you repeat a value:

Python

print("{0} scored {1}, so {0} wins".format("Ada", 9))
print("{name} is {age}".format(name="Ada", age=36))

Output

Ada scored 9, so Ada wins
Ada is 36

The format spec after a colon is identical to the f-string one:

Python

print("{:,.2f}".format(1234.5678))

Output

1,234.57

The % style

The oldest form, borrowed from C. You will meet it in old code and in logging:

Python

print("%s is %d" % ("Ada", 36))
print("%.2f" % 3.14159)

Output

Ada is 36
3.14

%s takes anything, %d an integer, %f a float. It is positional and easy to get wrong — one value needs a trailing comma to be a tuple — so do not start new code with it.

Lining things up

Width and alignment turn a loop into a table:

Python

rows = [("Ada", 36), ("Grace", 45), ("Katherine", 101)]
for name, age in rows:
    print(f"{name:<12}{age:>4}")

Output

Ada           36
Grace         45
Katherine    101

Pick a fill character by putting it before the alignment:

Python

print(f"{'Total':.<20}{42:.>6}")

Output

Total...................42

Centring and truncating

Python

print(f"|{'title':^20}|")
print(f"{'a very long value':.10}")

Output

|       title        |
a very lon

A bare number after the dot truncates a string, which is handy for keeping columns from blowing out.

Which to use

f-string for almost everything
str.format() when the template is not written where it is used
% when the surrounding code already uses it, and in logging calls

Test yourself

2 questions

When is str.format() a better choice than an f-string?

Show the answer

When the template is a variable rather than written where it is used — An f-string is interpolated where it is written, so it cannot take a template loaded from elsewhere.

What does f"{name:<12}" do?

Show the answer

Pads name on the right to at least 12 characters — The < aligns left, so the padding goes on the right. Use > to right-align.

Next chapter

Escape Characters

Backslashes for newlines, tabs and quotes, and the raw strings that switch them off.