Python Strings

Python Strings: Slicing, Methods, and F-Strings

A Python string (str) is an immutable sequence of Unicode characters wrapped in single quotes '...', double quotes "...", or triple quotes """...""". Every text value from user names to JSON payloads is handled as a str.

Because strings are immutable, string methods never edit the original text in place. Instead, methods return a brand-new string that you assign back to a variable.

Indexing and Slicing Strings

Each character sits at a zero-based index from left to right, or a negative index starting at -1 from the end. Slice notation text[start:stop:step] extracts substrings cleanly.

sku = "PROD-2026-XL"

prefix = sku[:4]       # "PROD"
year = sku[5:9]        # "2026"
size = sku[-2:]        # "XL"
reversed_sku = sku[::-1]

print(prefix, year, size, reversed_sku)

Everyday String Methods and F-Strings

Combine strip(), lower(), replace(), and split() to clean messy text, and use f-strings (f"...") to embed expressions directly inside strings.

raw_email = "  [email protected] 
"
clean_email = raw_email.strip().lower()
user, domain = clean_email.split("@")

print(f"User: {user} | Domain: {domain}")

Common Mistakes to Avoid

• Trying to change one character by index: word[0] = "B" raises TypeError; build a new string with slicing instead.
• Forgetting to save method results: Calling name.strip() without name = name.strip() leaves name unchanged.
• Concatenating hundreds of strings with + in a loop: Collect pieces in a list and join them once with "".join(parts).

Frequently Asked Questions

Is there any difference between single and double quotes in Python?

No. 'hello' and "hello" create identical str objects. Use double quotes when the text contains an apostrophe like "It's ready".

How do I check if a substring exists inside a string?

Use the in operator, such as "error" in log_line.lower(), which returns True or False.

What is a raw string (r"...")?

Prefixing a string with r disables backslash escapes, which keeps Windows file paths and regular expressions readable.