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Strings and Formatting

Python has many string operations, but the ones that are truly common and worth building muscle memory for aren't that many. This page is split into two parts:

  • How to handle strings themselves
  • How to format strings for output

Basic Operations​

text = "hello"

print(len(text))
print(text[0])
print(text[1:4])
print(text.upper())
print(text.replace("ll", "LL"))

Strings are immutable objects, so these methods return new strings and don't modify the original in place.

Common Concatenation Methods​

first = "hello"
second = "world"

print(first + " " + second)
print(" ".join([first, second]))

When repeatedly concatenating multiple fragments, join() is generally preferred.

f-string​

This is my default recommended formatting approach:

name = "alice"
score = 95.1234

print(f"{name}: {score:.2f}")

Advantages:

  • High readability
  • You can write expressions directly
  • The relationship with variables is very clear

str.format()​

name = "alice"
score = 95.1234

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

Still commonly seen today, but unless you need backward compatibility, I prefer using f-strings directly.

Legacy % Formatting​

name = "alice"
age = 20

print("%s is %d years old." % (name, age))

Still seen in older projects, but new code generally doesn't prioritize this approach.

Common Format Controls​

value = 3.14159

print(f"{value:.2f}")   # 3.14
print(f"{value:8.2f}")  # width 8, 2 decimal places
print(f"{42:04d}")      # 0042

A frequently encountered detail: round() and formatting are not the same thing​

  • round(x, 2): Changes the numeric value
  • f"{x:.2f}": Emphasizes output format

If you need to "strictly display two decimal places", prefer formatting the string.

Practices I recommend​

  • Default to f-strings first
  • Prefer join() for multi-part string concatenation
  • When you need to look up methods, see Common String Methods