Names

• Values can be assigned to names to make referring to them easier.
• A name can only be bound to a single value.
• Demo One way to introduce a new name in a program is with an assignment statement（赋值语句）
• Statements affect the program, but do not evaluate to values

To execute an assignment statement:

1. Evaluate the expression to the right of =.
2. Bind the value of the expression to the name to the left of = in the current environment.

Functions

• Functions allow us to abstract away entire expressions and sequences of computation
• They take in some input (known as their arguments) and transform it into an output (the return value)
• We can create functions using def statements. Their input is given in a function call, and their output is given by a return statement.
• 

Drawing Environment Diagrams

• Option 1: Python Tutor (tutor.cs61a.org)
  • ○ Useful for quick visualization or for environment diagram questions
• Option 2: PythonAnywhere (editor.pythonanywhere.com)
  • ○ Includes an integrated editor/interpreter
  • ○ Good for more complicated code or if you want to debug
  • ○ Developed by Rahul Arya

Summary

• Programs consist of statements, or instructions for the computer, containing expressions, which describe computation and evaluate to values.
• Values can be assigned to names to avoid repeating computations.
• An assignment statement assigns the value of an expression to a name in the current environment.
• Functions encapsulate a series of statements that maps arguments to a return value.
• A def statement creates a function object with certain parameters and a body and binds it to a name in the current environment.
• A call expression applies the value of its operator, a function, to the value(s) or its operand(s), some arguments

Control

None Indicates that Nothing is Returned

• The special value None represents nothing in Python
• A function that does not explicitly return a value will return None
• Careful: None is not displayed by the interpreter as the value of an expression

Pure Functions & Non-Pure Functions（print VS return）

Pure function是指在没有副作用的情况下，根据输入参数返回确定性输出结果的函数。这种函数在不同的时间和不同的上下文中都会返回相同的结果。

• Pure function不改变任何外部状态，也不依赖于外部状态，因此可以被安全地并发执行。

Non-pure function是指在执行过程中会产生副作用（例如改变外部状态、修改全局变量或读取用户输入等）的函数。

• 这种函数的输出结果不仅取决于输入参数，还取决于函数执行时的环境和外部状态。非纯函数难以测试、难以调试，也不便于构建可靠的、可维护的软件系统。

• 纯函数：仅输出返回值
• 非纯函数：有其他结果，如 print（输出（显示）值，并返回 none）

>>> print(print(1), print(2)) 
1 
2 
None None

Condition statement

Boolean context is any place where an expression is evaluated to check if it’s a True value or a False value

• False values in Python: False, None, 0, ‘’ (more to come)
• True values: everything else

Boolean Expressions

• Boolean expressions contain special operators and, or, not

Iteration

斐波那契数列

Summary

• print vs None
  • None represents when an expressions doesn’t evaluate to a value
  • print displays values in the interpreter
• Control
  • Allow for the interpreter to selectively or repetitively execute parts of a program
• Iteration
  • A particular variant of control which is based in the idea of repeating operations to compute a valu