Control

>>> print(None, None)
None None
>>> print(print(1), print(2))
1
2
None None
# execute two print expressions
>>> var = print(1)
>>> var + 4
Type Error! Nonetype and int 

None indicates nothing is returned

• the special value none represents nothing
• a function that doesn't explictly return a value will return None
• None will not be displayed in interactive python intepreter

Function

pure function: just return values

abs, pow. They just calculate the value

non-pure functions: have side effects

print(1). It's return value is None, but display 1 additionally

divide

• true divide: use slash /
• integer divide: //
• mod: %

from operator import truediv, floordiv, mod
truediv(2013, 10)	# 201.3
floordiv(2013, 10)	# 201
mod(2013, 10)		# 3

# more return value of a function
def divide_exact(n, d):
    '''return the quotient and remainder of dividing n by d
    
    # some examples
    >>> q, r = divide_exact(2013, 10)
    >>> q
    201
    >>> r
    3
    '''
    return n // d, n % d
	# from operator import floordiv, mod
    # return floordiv(n, d), mod(n, d)
quotient, remainder = divide_exact(2013, 10)
# write in a line instead of two lines

How can we run interactive interpreter in bash?

python -i doc.py

it will execute the document and python prompt appears in the last line.

you can use the func inside

How to run doctest in bash

def func():
    '''this is a function
    
    >>> a = func()
    >>> a
    5
    '''
    # if you type\
    # >>> a
    # 3
    # it will go wrong: expected 3, but got 5
    return 5

python -m doctest document.py means running the py, if it's right, nothing is displayed

python -m doctest -v document.py

please follow the standard above. don't type -m -v doctest

Conditional Statements

a statement is executed by the interpreter to perform an action

• assignment and def statement

If statement

def absolute_value(x):
    '''Return the absolute value of x.'''
    if x < 0:
        return -x
    elif x == 0:
        return 0
    else:
        return x

# more concise code style
if x > 3:
    return True
else:
    return False

Value

• False value: False, 0, '', [], None(more to come)
• True value: Anything else(True)

read section 1.5.4

Iteration: While and For

always use pythontutor

Instance

prime factorization
def prime_factor(x):
    '''print the prime factor of x in non-decreasing order
    
    >>> prime_factor(8)
    2
    2
    2
    >>> prime_factor(6)
    2
    3
    '''
    while x > 1:
        k = smallest_prime_factor(x)
        x = x // k
        print(k)
        
def smallest_prime_factor(n):
    '''Return the smallest prime factor of n'''
    k = 2
    while n % k != 0:
        k += 1
    return k

Boolean

• True, False
• Not

Not has the highest priority, then and, or has the lowest priority

short-circuit

>>> True or 1/0
True
# instead of printing ZeroDivisionError, it outputs True due to short-circuiting

Priority

and or always return the last value they evaluate, no matter whether they are booleans

>>> True and 'hello'
'hello'

Debugging

Instance

Traceback (most recent call last):
  File "<pyshell#29>", line 3 in <module>
    result = buggy(5)
  File <pyshell#29>", line 5 in buggy
    return f + x
TypeError: unsupported operand type(s) for +: 'function' and 'int'

Traceback Messages

The first line

File "<file name>", line <number>, in <function>

• File name: the file contains the problem
• Number: the line number that causes the problem
• Function

The second line

display the actual line of code that makes the function call

Error Messages

<error type>: <error message>

• Error type: like SyntaxError, TypeError
• Error message: detailed description of what exactly caused the problem

Debugging Techniques

Running doctests

python -m doctest <file>.py tell you which doctests you failed

python -m doctest file.py -v also tells which doctests you passed

Using print statement

print('DEBUG: result is', result)

Using specific prefix string 'DEBUG: ' allows cs61a ok to ignore this print statement

Interactive Debugging

python -i file.py enables you to debug in an interactive REPL

PythonTutor

python ok -q question --trace 打开网页

不过不知道为什么，打开的是空白网页

Long-term Debugging

debug = True
if debug:
    print('DEBUG: i is', i)
# when we don't want to see the debugging output, we can turn debug to False

Assert

assert isinstance(x, int)

Error types

SyntaxErrors: Before executing any code, python will check SyntaxErrors

各种思路

Q&A

• -what non-pure function do aside from printing, like changing global variables

• a function can not only changing the global variable, but also changing another function, even itself

• intrinsic name and bound name

• >>> abs
  <built-in function abs>
  >>> f = abs
  >>> f
  <built-in function abs>
  >>> abs = 4
  >>> f
  <built-in function abs>
  

• function call

• def f
  def g
  f(g)
  
  # it first call g, evaluate the operand expression
  # then it calls f. g comes first, you should evaluate the operands first.
  # it needs to bind g to a name or something
  

• difference between frame and environment

• for every frame there's an environment, for every environment starts with a first frame. An environment is a sequence of frames

• floor, ceiling and round operator

• floor discards decimal and ceiling get up and round operator toggles numbers at point five

• True of False

• in python, any number except zero is true, any string except '' is true

if 'hello':

​ print(7)

My own questions

short-circuit(have solved)

# print two pairs of single quote
>>> True or ''hello''
  File "<stdin>", line 1
    True or ''hello''
              ^
SyntaxError: invalid syntax
# why?

>>> True or ''hello''
Error!
# it will first check the syntax, whether its syntax is right
# translate the source code(src) into byte code