CS61A lect3

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
example:print(print(1), print(2))
1
2
None None

Miscellaneous Python Features
interept：python3 -i xx.py
multiple return values
doctests:
write inputs in the 注释！
for example:
""">>>q, r=divide_exact(5, 3)
>>>q
201
>>>r
3
"""
python3 -m doctest -v xx.py
Compound Statements
Execution Rule for a sequence of statements:
• Execute the first statement
• Unless directed otherwise, execute the rest
Conditional Statements
Execution Rule for Conditional Statements:
Each clause is considered in order.

  1. Evaluate the header’s expression.
  2. If it is a true value, execute the suite & skip the remaining clauses.

Syntax Tips:

  1. Always starts with “if” clause.
  2. Zero or more “elif” clauses.
  3. Zero or one “else” clause, always at the end.
    Boolean Contexts
    False values in Python: False, 0, ‘’, None
    True values in Python: Others
    While Statements
    Execution Rule for While Statements:
  4. Evaluate the header’s expression.
  5. If it is a true value, execute the (whole) suite, then return to step 1.