Intuition The ONE core ideaBehavioral patterns are all about how separate objects talk without being glued together : instead of one object reaching inside another and calling its methods directly, we put a thin agreed-upon shape (an interface) between them so either side can change alone. If you understand what an object , a method call , an interface , and "programming to that interface" mean, every pattern in the parent note is just a clever rearrangement of those four bricks.
The parent note throws around words like object , method , interface , delegate , subscribe , runtime , generator , hook , and self. If any of those feel fuzzy, this page builds them from nothing. Read top to bottom — each brick rests on the one above it.
An ==object is a bundle that holds some data (fields) and some behavior (methods) together in one box.== The data is what it knows ; the methods are what it can do .
Think of a physical box with two drawers. The top drawer holds sticky notes (the data — e.g. text = "hello"). The bottom drawer holds tools (the methods — e.g. append(x)). You never fiddle with the sticky notes directly from outside; you use the tools.
Why the topic needs it: every behavioral pattern is a story about boxes handing work to other boxes . Observer has subject-boxes and observer-boxes; Command wraps a job in a box. No boxes, no patterns.
Definition Field (attribute)A ==field is one named slot of data living inside a particular object.== In the parent code, self._observers = [] creates a field called _observers holding an empty list.
self
self is the word a method uses to say =="this very object I'm attached to" ==. When you write self._observers, you mean the observer-list belonging to me, this specific box .
self must exist
The same method code is shared by every box of that type. When it runs, it needs to know which box's drawers to open. self is the pointer that says "these ones". Look at figure s01: the arrow labelled self points back at the box currently running the method.
self is a keyword I must never touch."
No — self is just the conventional name of the first parameter every method receives automatically. You could technically call it anything; everyone calls it self so others can read your code.
A method is a function that lives inside an object and can read/change that object's fields via self.
Definition Method call — the notationx.do(a)
The dot . means "reach into" . So s.subscribe(o) reads as: ==take the box named s, open its subscribe tool, and hand it o.== The thing before the dot is who does it ; the thing after is what they do ; the stuff in (...) are the arguments (the inputs).
Intuition Why the dot matters for behavioral patternsA method call is one object asking another to do something. The entire question behavioral patterns answer — "how do objects talk?" — is literally "who is allowed to write a.b(), and what should b be?" Command turns the call itself into a stored object; Observer turns one call into many; Strategy swaps which method the dot lands on.
A class is the blueprint / cookie-cutter ; an object is one cookie stamped from it . class Logger: describes the shape; Logger() bakes an actual box you can use.
One cutter, many identical-shaped cookies — but each cookie can carry different sprinkles (different field values). The parent's Draft() and Published() are two cookies from two different cutters.
Why the topic needs it: patterns like State and Template Method work by making new classes (new cutters) instead of adding if branches. "Adding a status = adding a class" (parent note) only makes sense once class ≠ object is clear.
This is the single most important brick. Read slowly.
An ==interface is a promise about which methods exist , without saying how they work.== "Anything that has an update(data) method counts as an Observer." It is a contract of shape , not of behaviour.
Intuition The picture: a wall socketA wall socket is an interface. It promises "two holes, this spacing, this voltage." A lamp, a charger, a toaster all fit — the wall does not know or care which. In figure s03 the socket is the same; the plugs behind it differ.
Definition "Program to the interface, not the implementation"This slogan means: ==write code that only relies on the socket shape (o.update(...)), never on which appliance is plugged in.== The parent's Subject loops o.update(data) without ever knowing o is a Logger. That ignorance is the decoupling.
Why the topic needs it: every behavioral pattern hangs on this. The subject talks to observers through an interface; the context talks to strategies through one; the invoker calls command.execute() through one. See SOLID Principles (the "D" — depend on abstractions) and Polymorphism for the deeper machinery.
==One method name (execute, update) behaving differently depending on which object receives it.== Same dot-call, different box, different outcome.
You say "speak!" to a dog and a cat. Identical command, different sound. The caller said one word; the receiver decided the behaviour. That is why a for o in observers: o.update(...) loop can fan out to a logger, a chart, and a status bar with one line of code.
Deep dive lives at Polymorphism . Behavioral patterns are essentially polymorphism organised into recurring shapes .
To delegate is to receive a request and immediately hand it to another object to actually do . The parent's Context "delegates to strategy.execute()": it does no real work, it just forwards.
A receptionist takes your question and forwards it to the right department. The receptionist is the Context ; the department is the Strategy or State . Chain of Responsibility is pure delegation: each handler either does the job or passes it down the line.
Common mistake "Delegation = inheritance."No. Inheritance = "I am a kind of X" (a Sales report is a Report). Delegation = "I hold an X and ask it to do things" (a Context has a strategy). Template Method uses inheritance; Strategy uses delegation. Same effect, opposite mechanism — the parent's "Strategy vs Template Method" note rests entirely on this distinction.
==Runtime is while the program is actually running ==, as opposed to when you were typing the code. "Choose the strategy at runtime" means the decision happens live , based on data the user just gave — not baked in when you wrote the file.
The whole selling point of these patterns is "swap behaviour while running" : subscribe a new observer mid-execution, pick sort order from a dropdown the user just clicked. If everything were decided at write-time, a plain if would do.
Definition First-class functionA function you can store in a variable, pass as an argument, and return — treated like any other value. The parent's Context(sorted) passes the function sorted itself as the strategy. In Python a function already is a strategy object , so the minimal Strategy needs no class.
yield
==yield produces one value, pauses, and remembers where it stopped==; the next request resumes right after. That "remember where I paused" is an iterator's traversal state. The parent's Countdown uses yield i so it hands out 3, then 2, then 1, one at a time.
A ticket dispenser: press once → one ticket, and it silently advances to the next number. It doesn't hand you the whole roll (the internal structure stays hidden) — exactly Iterator's promise: traverse without exposing internals .
Object = data plus behavior
First-class functions and generators
Read it as: boxes and their fields make method calls possible; classes make interfaces possible; interfaces unlock polymorphism and delegation; those two, plus inheritance and runtime choice, are the raw material every behavioral pattern rearranges.
Cover the right side and answer out loud before revealing.
An object bundles which two things? Data (fields) + behavior (methods) in one box.
What does self refer to inside a method? The specific object that method is currently running on.
In s.subscribe(o), what do the three parts mean? s = who does it, subscribe = the method, o = the argument handed in.
Difference between a class and an object ? Class = blueprint/cutter; object = one instance stamped from it.
Define interface in one line. A promise about which methods exist, with no promise about how they work.
What does "program to the interface" force you to ignore? Which concrete class is behind the method — only the method shape is used.
Polymorphism in one sentence?The same method call gives different results depending on which object receives it.
Delegation vs inheritance ?Delegation = has-a object it forwards to; inheritance = is-a kind of a base class.
Why does "choose at runtime " matter here? Behavior is picked live from current data, not hard-coded when writing the file.
What does yield remember? Where it paused — that saved position is the iterator's traversal state.
Which pattern mechanism does Template Method use, and which does Strategy use? Template Method = inheritance; Strategy = delegation/composition.
See also: Design Patterns — Creational , Design Patterns — Structural , Publish-Subscribe & Event-Driven Architecture (Observer at scale), Undo-Redo Systems (Command in practice), and back to the parent Hinglish version .
