DSC 20 Lecture 7

Assert Statements in Python

Asserts and Exceptions

Assertions should be used to check something that should never happen, while an exception should be used to check something that might happen.

Assertions

assert is a keyword in Python
Syntax:

assert "condition"  
assert "condition", "optional error message"  

If condition is False raise AssertError exception

• Assertions can be disabled at runtime using parameters, and are disabled by default, so don’t count on them except for debugging purposes.
• Using -O    ( O is for Optimized)
• Assertions are used for debugging purposes only.

type() vs. isinstance()

name = "Carter"  
print(type(name) == str) # True  
print(isinstance(name, str)) # True  

Note

print(isinstance(1, int))  
print(isinstance(True, int)) # True, because booleans are a subclass of integers  

In this class, we will use assert, type(), isinstance() to validate function parameters

This is not recommended in production code.

callable()

Check if something is a function

def my_func():  
	return  
print(callable(my_func))  

all()

The all() function returns True if all items in an iterable are true, otherwise it returns False.

lst1 = [1, 2, 3, 4]  
lst2 = [1.1, 2, 3, 4]  
assert all([isinstance(elem, int) for elem in lst1]), 'each elem is not int in lst1'  
assert all([isinstance(elem, int) for elem in lst2]), 'each elem is not int in lst2'  

any()

The any() function returns True if at least one item in the iterable is true, otherwise it returns False.

Can't check for integers in list before checking if it is actually a list

Realistic example of assert

def apply_discount(product, discount):  
	price = int(product['price']*(1.0 - discount))  
	assert 0 <= price <= product['price']  
	return price  

General rules

Don’t use asserts for data validation

• can be disabled
• homeworks: for grading/learning purposes
  Asserts that never fail

Strings

Representing data

Representing language

Representing program

Escape characters

try:  
	print("Hello "Carter"")  
except Exception as e:  
	print(e)  
print("Hello \"Carter\"")  
print("Hello \\Carter\\)  

\a Bell escape character

Makes noise?
Originated to alert programmers when their programs finished

String operations

str * int: str written int times
str[int]: character at int index of str
str[int:int]: substring from int index to int index

Slicing str[int:int:int]

Can omit indices, use negative indices, specify step

test_str = 'abcdefghijklmnopqrstuvwxyz'  
print(test_str[::-2])  

String modification

str[16] = "" ‘str’ item does not support item assignment (immutable)
str = str[:16] + " " + str[17:]

str.replace()

newStr = "The exam is coming."  
print(newStr.replace("coming", "cancelled")) # Does not modify newStr  
print(newStr)  
exam = newStr.replace("coming", "cancelled")  
print(exam)