Diagrams

To keep up what is going on within the python interpreter when it execute an program we typed in.

Visualize here!

Frame :

“帧”.
It records the value of all the variables in this time (last sentence is executed while next not yet)

Every expression is evaluated in the context of an environment.

The global frame alone.
A local frame, then the global frame.
Or, we have Higher-Order Function, local, parent, parent…then global.

All in all, find its parent.

Most important two things :

An environment is a sequence of frames.
A name evaluates to the value bound to that name in the earliest frame of the current environment in which that name is found.
E.g., to look up some name in the body of the square function:
Look for that name in the local frame.
If not found, look for it in the global frame.
(Built - in names like “max” are in the global frame too, but we don’t draw them in environment diagrams.)

Multiple Environment

A call expression and the body of it are evaluate in different environment.

The Environment of Higher-Order Function

The rule above is still effective.

The Environment of Nested Definition

def add_maker(n):
	def adder(k):
		return n + k
	return adder

This is Nested Function. When the function created in it is returned, it will bind to a name in the environment where we call the adder_maker function. Then we can get access to this adder function.

So when we return this function, it is a special category of Higher-Order Function.

Lecture 5: 27:04

When a function is defined:

Create a function value: func <name>(<formal parameters>) [parent=<parent>]
Its parent is the current frame.
Bind <name> to the function value in the current frame

When a function is called:

Add a local frame, titled with the <name> of the function being called.
Copy the parent of the function to the local frame: [parent=<label>]
Bind the <formal parameters> to the arguments in the local frame.
Execute the body of the function in the environment that starts with the local frame.

Summary:

Every user-defined function has a parent frame (often global).
The parent of a function is the frame in which it was defined.
Every local frame has a parent frame (often global).
The parent of a frame is the parent of the function called.

Follow the frame according the priority, so we can find the correct value bound to the name we search for, writing down frame correctly.

So its status will remain. What if there’s a variable in it, the outer function?

Function defined within another function bodies are bound to a name in a local frame. For variables in outer one, we could use nonlocal to use that variable. When use it and return it, it is called closure.

Attention: The parent environment of a function is where it was defined rather than where it is called, so we have a nonlocal keyword in a function defined in a function, which indicate to use the variable in its parent environment - The Higher-Order Function where it is defined.

The Environment of Recursive Functions

The same function fact is called multiple times.
Different frames keep track of the different arguments in each call.
What n evaluates to depends upon which is the current environment.
Each call to function solves a simpler problem than the last.
And when a function is called, we can’t continue next statement until it is returned.

シリウスの砂

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