What are Instance Variables and Methods?

What you need to know

Instance vs class variables

1class ApiClient:2    MAX_RPM = 60                         # class variable: one value, shared34    def __init__(self, api_key):5        self.api_key = api_key           # instance variable: per object6        self.calls = 078    def call(self):                      # instance method9        self.calls += 110        return self.calls <= self.MAX_RPM1112a, b = ApiClient("key-a"), ApiClient("key-b")13a.call(); a.call(); b.call()14print(a.calls, b.calls, ApiClient.MAX_RPM)   # 2 1 60

How attribute lookup works

When you read a.MAX_RPM, Python looks in the instance's own __dict__ first, then in the class, then in parent classes. If none has it, you get AttributeError.

Assigning through the instance, a.MAX_RPM = 10, does not change the class variable. It creates a new instance variable on a that hides the class one for a only. Mutating through the instance is different: a.some_list.append(x) changes the one shared list.

The three kinds of method

1class Tokenizer:2    def __init__(self, vocab):3        self.vocab = vocab45    @classmethod6    def from_words(cls, words):7        return cls({w: i for i, w in enumerate(words)})89    @staticmethod10    def normalise(text):11        return text.lower().strip()1213tok = Tokenizer.from_words(["refund", "upi"])14print(tok.vocab, Tokenizer.normalise("  UPI "))   # {'refund': 0, 'upi': 1} upi

A real-life example

A gateway calls an LLM on behalf of many customers and tracks request timestamps per API key. The first version put the log in the class body:

1class RateLimiter:2    log = []                              # BUG: one list for EVERY instance34    def __init__(self, key):5        self.key = key67    def record(self, ts):8        self.log.append(ts)               # appends to the shared class list9        return len(self.log)1011a, b = RateLimiter("customer-a"), RateLimiter("customer-b")12for t in range(5):13    a.record(t)14print(b.record(99))                       # 6 -> customer B "used" A's requests

Customer A's burst counted against every customer, and quiet customers started getting throttled. The fix is one line: create self.log = [] inside __init__, so each limiter owns its list. Keep class variables for values that really are shared, such as MAX_RPM = 60.

Follow-up questions to expect

• "What happens if you assign to a class variable through self?" — You create an instance variable with the same name that shadows the class variable for that one object; the class value is unchanged.
• "When do you use @classmethod vs @staticmethod?" — @classmethod when you need the class, typically to construct instances; @staticmethod for a helper that needs neither class nor instance.
• "Where are instance variables stored?" — In each object's __dict__, unless the class uses __slots__ to save memory.