Strings and f-strings

Text in quotes

A string is a piece of text. Picture a row of letter tiles: each tile is one character, and the row is the string. Single and double quotes work the same way, so pick the one that doesn't clash with the text inside:

headline = "It's our biggest sale"
quote = 'She said "worth every penny"'
print(headline)
print(quote)

Three handy tools:

• + joins strings end to end,
• * repeats them,
• len() counts the characters, spaces and punctuation included.

first = "Spring"
second = "Sale"
print(first + " " + second)
print("-" * 20)
print(len("Spring Sale"))

Indexing and slicing

Every character in a string has a position, called its index. Counting starts at 0, not

1. Negative indexes count from the end: -1 is the last character.

• code[0] is the first character.
• code[-1] is the last.
• code[2:6] is a slice: from index 2 up to, but not including, index 6.
• Leave out a side to go to the edge: code[:8] is the first 8 characters, code[9:] is everything from index 9 on.

Drag the handles below and watch the expression and its result change.

Campaign codes, SKUs and UTM values often pack several facts into one string. Slicing pulls them apart:

code = "SUMMER25-EMAIL"
season = code[:8]
channel = code[-5:]
print(season)
print(channel)
print(code[0], code[-1])

Asking for an index past the end, like code[50], raises an IndexError. Slices are more forgiving: code[:50] gives you the whole string.

word = "Newsletter"
print(word[1:4])

String methods

A method is a function that belongs to a value. You call it with a dot after the value: subject.lower(). Strings come with dozens of methods. These are the ones you'll use most:

Strings can't be changed in place. A method gives you back a new string, so store the result if you want to keep it. You can also chain methods. They run left to right:

raw = "  Spring SALE Launch  "
clean = raw.strip().lower()
print(clean)
print(raw)               # the original is unchanged
print(clean.replace("sale", "event"))

The in operator asks whether one string appears inside another. It's case-sensitive, so change everything to lowercase first when that matters:

subject = "Last chance: FREE shipping ends tonight"
print("free" in subject)
print("free" in subject.lower())

If you've done the numbers lesson, here's the fix it promised for numbers with commas: int("1,200".replace(",", "")) gives 1200.

tag = "  Email  "
tag.strip()
print(len(tag))

Splitting and joining

.split() breaks a string into pieces wherever a separator appears. It gives you back a list of strings: the values inside square brackets. Lists get their own lesson next. For now, read ['seo', 'email', 'ads'] as "three strings, in order".

.join() does the reverse. It's called on the separator you want between the pieces:

tags = "seo, email, ads".split(", ")
print(tags)
print(" | ".join(tags))

words = "write three short headlines".split()   # no separator: split on any whitespace
print(words)

f-strings: text built from values

Joining text with + gets clumsy fast, and it fails if one of the pieces is a number. Think of a form letter with blanks to fill in. An f-string is a string with an f before the opening quote. Anything inside curly brackets { } is worked out and dropped into the text:

product = "Midweight Hoodie"
audience = "college students"
count = 3
prompt = f"Write {count} ad headlines for the {product}, aimed at {audience}."
print(prompt)

That's exactly how you'll build prompts from data later: a template with blanks, filled in from variables.

After the value you can add a colon and a format spec to control how numbers look:

spend = 3120.5
impressions = 182340
clicks = 5470
ctr = clicks / impressions
print(f"Spend: ${spend:,.2f}")
print(f"{impressions:,} impressions, {clicks:,} clicks, CTR {ctr:.1%}")
print(f"Cost per click: {spend / clicks:.2f}")

The braces can hold any expression, such as the division in the last line.

Special characters and multi-line text

A backslash inside a string starts an escape sequence, a way to type characters that are otherwise awkward:

print("Subject: Spring sale\nPreview: 30% off everything")
print("Name:\tDana")

For longer text, such as a prompt template, use triple quotes. The string can span several lines, and the line breaks are kept:

product = "Canvas Tote"
prompt = f"""You are a copywriter for an online store.
Write one Instagram caption for the {product}.
Keep it under 20 words."""
print(prompt)

raw_subject = "   LAST CHANCE: 30% off ends tonight!   "

subject = raw_subject.strip()
softer = subject.capitalize()
preview = subject[:20]

print(f"Cleaned:   {softer}")
print(f"Length:    {len(subject)} characters")
print(f"Preview:   {preview}...")
print(f"Discount?  {'%' in subject}")

Methods give you a new string

.upper(), .strip() and .replace() don't change the original text. They hand back a new string, and if you don't keep it, it's gone.

name = "  ada "
name.strip()
print(repr(name))
name = name.strip()
print(repr(name))

Text can't be changed in place in Python, so the only way to "update" a string is to save the result. Note too that in and == care about capitals: "Sale" in "summer sale" is False. Comparing cleaned-up text, such as text.lower(), avoids that surprise.

Check before you slice

A slice stops before its end index, so text[0:3] gives three characters. And an f-string needs its f, or the braces print as they are.