Python Specification: use with assert

What is assert

　　assert syntax:

assert_stmt ::=  "assert" expression ["," expression]

　　Example:

assert 1 == 2,  'assertion is wrong'
Traceback (most recent call last):
  File "assert.py", line 4, in <module>
    assert 1 == 2,  'assertion is wrong'
AssertionError: assertion is wrong

　　Equivalent to

if __debug__:
    if not expression1: raise AssertionError(expression2)

　　Assert role in the program, the code is doing some of the internal self-check. Use assert, it means you're OK. This condition will happen or will not happen.

　　Note: You can not use parentheses when assert

assert(1 == 2, 'This should fail')
# 输出
<ipython-input-8-2c057bd7fe24>:1: SyntaxWarning: assertion is always true, perhaps remove parentheses?
  assert(1 == 2, 'This should fail')

 assert usage

　　Examples of several common scenarios.

　　1. merchandise discount. Asked to enter the original price and discount, price after the discount is output 　　

def apply_discount(price, discount):
    updated_price = price * (1 - discount)
    assert 0 <= updated_price <= price, 'price should be greater or equal to 0 and less or equal to original price'
    return updated_price

　　If you assert conditions for False, will raise

apply_discount(100, 0.2)
80.0

apply_discount(100, 2)
AssertionError: price should be greater or equal to 0 and less or equal to original price

　　2. Background want to know the average selling price of each item, the algorithm is the total sales / Number of sales:

def calculate_average_price(total_sales, num_sales):
    assert num_sales > 0, 'number of sales should be greater than 0'
    return total_sales / num_sales

　　Joined assert statement, the number of sales provisions must be greater than zero.

　　3. Check whether the input list

DEF FUNC (INPUT):
     Assert the isinstance (INPUT, List), ' INPUT MUST BE type of List ' 
    # The following operations are based on the premise: input must List 
    IF len (INPUT). 1 == : 
        ... 
    elif len ( the INPUT) == 2 : 
        ... 
    the else : 
        ...

assert error example

 　　assert checks can be turned off, such as when running a Python program, adding -O This option will let assert failure. Therefore, once assert checking is turned off, user_is_admin () and course_exist () These two functions will not be executed. This can lead to serious problems:

def delete_course(user, course_id):
    assert user_is_admin(user), 'user must be admin'
    assert course_exist(course_id), 'course id must exist'
    delete(course_id)

　　For programs in some of the run-time error, or starting exception handling

def delete_course(user, course_id):
    if not user_is_admin(user):
        raise Exception('user must be admin')
    if not course_exist(course_id):
        raise Exception('coursde id must exist')
    delete(course_id)  

reference

 　　Geek Time "Python core technology and practical" column