CS61A: Environmental Diagrams

Environment Diagrams

• Used to keep track what goes on in the python interpreter.
• Memory that keeps track of bindings between names and values.
• Expressed in the form of pictures
• Operates in the form of frames. Each frame represents a subroutine, with the main program executing in the global frame.

Code	Frames
Statements and expressions	Each name is bound to a value
Arrows indicate evaluation order	Within a frame, a name cannot be repeated (collisions). Must be counded to one value

• Once a new binding is created, the name loses its original value.
• We can practice drawing the environmental diagrams at this link

Some Cool Syntax

• For built-in functions, the formal parameters are not displayed. Formal paramters are displayed for user-defined functions. Ex:

  from operator import mul
  def multiply(a,b):
    return a,b
  

  Environmental Diagram:

  Global Frame
    mul | func mul(...)
    multiply | func multiply(a,b)
  

• Local frames are named according to the name of the function they represent. Using the above example: ```python from operator import mul def multiply(a,b): return a,b

multiply(1,2)

Global frame // This is our global frame mul | func mul(…) multiply | func multiply(a,b)

multiply // This is a local frame a | 1 // Binding betwen argument name and value b | 2 return value | 2 // The return value is not a binding! This is annotation

# Assignment Statements
- Assignment statements change the bindings between names and values in frames
- Execution rule for assignment statements
    - The expressions are evaluated from left to right
    - Once evaluated, the new values are then binded to the values on the left.

```python
a = 1               # Standard assignment
b = 2
b, a = a + b, b     # First a+b is evaluated to 3, then b evaluates as b, then 3 is assigned to b, and 2 is assigned to a.

• Sample environmental diagram for the following code:

  f = min
  f = max
  g, h = min, max
  max = g
  max(f(2,g(h(1,5),3)),4)
  

  Global frame
  f | function max // Max function has 2 names but there is still only 1 max function
  g | function min
  h | function max
  max | function min
  min | function min