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Part One
Functions vs. Methods
You have already used print() and len(). These are functions — standalone commands that you call by writing their name followed by parentheses, with an input inside: len("hello").
A method is different: it belongs to a specific piece of data and is called using dot notation — the data first, then a dot, then the method name: "hello".upper(). Think of a method as something the data knows how to do to itself.
Rule of thumb: If it starts with the data and a dot —
something.method() — it's a method. If the data goes inside the parentheses — function(something) — it's a function.
Part Two
Built-in Functions
Python ships with dozens of functions that are always available. Here are the ones you will reach for most often.
len() — the length of things
len() returns the number of items in a list, or the number of characters in a string.
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type() — what kind of thing is this?
type() tells you what data type a variable holds. Useful when something isn't behaving as expected.
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int(), float(), str() — converting between types
These functions convert a value from one type to another. This is essential when, for example, you read a number that is stored as text and need to do arithmetic with it.
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round() — rounding numbers
round(number, digits) rounds a number to the given number of decimal places. If you omit the second argument it rounds to the nearest whole number.
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max() and min() — largest and smallest
These work on a list of numbers (or strings) and return the largest or smallest value.
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sum() — total of a list
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Part Three
String Methods
Strings in Python come loaded with methods. Because strings are so common in real programs — names, addresses, messages, file contents — these methods save enormous amounts of work.
.upper() and .lower() — changing case
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.title() — title case text
.title() makes the first letter of each word uppercase and the rest lowercase.
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.strip() — removing extra whitespace
Real-world data often has unwanted spaces at the start or end. .strip() removes them.
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.replace() — substituting text
.replace(old, new) returns a new string with every occurrence of old swapped for new.
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.split() — breaking a string into a list
.split(separator) cuts the string wherever it finds the separator and returns a list of the pieces. If you omit the separator, it splits on any whitespace.
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.startswith() and .endswith() — checking beginnings and endings
These return True or False, making them ideal for use inside if statements.
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.count() — counting occurrences
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Slicing strings — taking parts of text
You can extract part of a string with slicing: text[start:end]. It starts at start and stops before end.
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.camel() — making camelCase (custom helper)
Python does not include a built-in .camel() string method, but you can make the same result with a small helper function.
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Methods do not change the original. Strings in Python are immutable — methods always return a new string and leave the original untouched. If you want to keep the result, assign it to a variable:
clean = name.strip().
Part Four
List Methods
Lists also have their own set of methods. Unlike string methods, several list methods modify the list in place — they change the original directly, rather than returning a new one.
.append() — adding an item
You have seen this before. It adds one item to the end of the list.
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.sort() — sorting in place
.sort() rearranges the list alphabetically (for strings) or numerically (for numbers). It modifies the list directly.
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.count() — counting occurrences in a list
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Removing items — .remove(), .pop(), and del
There are different ways to remove from a list. .remove(value) removes by value, .pop(index) removes by position and returns the removed item, and del deletes by index.
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Putting it together
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In place vs. returning a new value:
.sort() and .append() modify the list directly and return nothing. .upper(), .strip(), .replace() leave the original unchanged and return a new value. When in doubt, print the result to see what you got.
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