W WolfCode · CSC 141

Chapter · Variables & Types

Strings & f-strings

字符串与 f-string

≈ 20 min · python-strings · Open in WolfCode

A string is a sequence of characters. f-strings — Python's modern way to weave values into text — are the only string formatting you really need in this course.

Strings: text in quotes

A string is any sequence of characters wrapped in quotes. Python accepts either single quotes '...' or double quotes "..." — pick one and be consistent. Use the other style when the string itself contains a quote.

Python · runnable

a = 'hello'
b = "hello"
print(a == b)            # → True. Quote style doesn't change the value.

# When the string contains a quote, use the other style to avoid escaping:
song = "Don't Stop Me Now"
quote = 'He said "ok"'
print(song)
print(quote)

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Python · runnable

print("line one\nline two")
print("col1\tcol2\tcol3")
print("She said \"hi\"")

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+ glues strings, * repeats them

The same + you use for numbers also concatenates strings — it joins them end-to-end. The * operator repeats a string. Both operands of + must be strings, though.

Python · runnable

first = "Ada"
last  = "Lovelace"
print(first + " " + last)        # → Ada Lovelace
print("=" * 30)                  # → ==============================
print("ha" * 4)                  # → hahahaha

# This one fails. Read the error:
# print("Hi " + 5)

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f-strings: the right way to build text

An f-string is a string literal with the letter f in front. Inside it, anything in {} is treated as a Python expression and replaced with its value at runtime. This is by far the most common way to format text in modern Python.

Python · runnable

name = "Alice"
age = 18

# The wrong way (works, but verbose and easy to mess up):
print("Hello, " + name + "! You are " + str(age) + " years old.")

# The right way:
print(f"Hello, {name}! You are {age} years old.")

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Inside the braces you can put any expression — not just a single variable:

Python · runnable

price = 4.50
qty   = 3
print(f"Subtotal: ${price * qty:.2f}")        # arithmetic + 2-decimal format
print(f"Average per item: ${price * qty / qty:.2f}")

name = "ada"
print(f"Title-cased: {name.title()}")         # method calls work too
print(f"Length: {len(name)} characters")

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The three formatting styles you may see

You'll find old code that uses %-formatting or .format(). They still work, but for this course, write f-strings.

All three lines produce the same output

name = "Alice"
age = 18

print("Hello, %s! You are %d years old." % (name, age))           # old: %-formatting
print("Hello, {}! You are {} years old.".format(name, age))       # 2010-era: .format()
print(f"Hello, {name}! You are {age} years old.")                 # modern: f-string ← use this

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Check yourself

What does this print? greeting = f"Hi {name.upper()}" print(greeting) — given name = "ada"

Check yourself

Which of these correctly puts the value of `qty` into the message?

Now you try

The exercise file 03-strings-and-fstring.py in WolfCode asks you to write a function build_greeting(name, age) that returns a greeting in the f-string style. The tests check both names and ages, so your function has to work for any input — don't hardcode "Alice".