Lecture 13: Exceptions and Modules

Craig Zilles (Computer Science)

May 1, 2020

https://go.illinois.edu/cs105fa19

CS 105

Today1. Exceptions• try/except, multiple exceptions, across functions

2. Modules• scripts, importing, paths

3. Larger Software Development• Stubs• Unit Testing

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What does this code do?prompt = 'type a number or q to quit\n'items = []while True:

item = input(prompt)if item == 'q':

breakitems.append(float(item))

value = sum(items) / len(items)print(value)

3

Where can an exception occur?prompt = 'type a number or q to quit\n'items = []while True:

item = input(prompt)if item == 'q':

breakitems.append(float(item))

value = sum(items) / len(items)print(value)

4

A

B

C

D

E

Exception Handling: Big Idea• Don't crash the program; gracefully handle user errors• Do it in a way that doesn't encumber the normal path• Exception handling (like objects) is complexity

management

try:# do potentially bad things

except:# handle bad things if they happen

5

Can differentiate between exceptions try:

# do potentially bad things

except ValueException as e:# handle ValueExceptions# e is bound to exception object

except ZeroDivisionError:# ...

except:# handle any other exceptions

6

Exceptions handed up "call chain"• Unhandled exceptions passed to caller

def f():x = 1/0print('hi')

def g():f()print('hello')

g()7

What gets printed?

A) Just hiB) Just helloC) Both hi and helloD) Nothing

Annoucements• 5% curve on Exam 2, 10% curve on Exam 3• Clicker points capped at 150• Lab after break: Data Analysis• Quiz 4: 12/5 - 12/7• Mix of Excel and Python content

• Final Exam• Almost all Python (can't run Excel in CBTF)

• Hiring Course Assistants for Spring: (see mass mail)• https://forms.gle/XqHiiphAYYJRXUHr8

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Modules• One of the main uses of computing is automation• Often you want to do similar things repeatedly

• Write a piece of code once and use it again and again• Put the code into a file (e.g., useful.py) • Import that file as a module• import useful• useful.py must be in Python's path

• Path is set of directories where Python will look for file• Most easily done by running Python where the file is

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useful.py defines useful_function

After:import usefulHow to call useful_function with "hi" as argument

A) function("hi")B) useful_function("hi")C) useful.function("hi")D) useful.useful_function("hi")E) None of the above

10

If execute.py includes:useful_function("during loading")

if __name__ == "__main__":

useful_function("WELCOME TO THE MACHINE")

What is printed if I run: import execute

A) NothingB) Just *** during loading ***C) Just *** WELCOME TO THE MACHINE ***D) Both *** during loading *** and

*** WELCOME TO THE MACHINE ***E) An error occurs

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If execute.py includes:useful_function("during loading")if __name__ == "__main__":

useful_function("WELCOME TO THE MACHINE")

What is printed if I run: python execute.py

A) NothingB) Just *** during loading ***C) Just *** WELCOME TO THE MACHINE ***D) *** during loading *** then

*** WELCOME TO THE MACHINE ***E) *** WELCOME TO THE MACHINE *** then

*** during loading ***

12

Larger software development• When building larger programs …

• Can't build it all at once: use modular design• Break it up into logical pieces • Those become modules, objects• Manages complexity

• Test each piece individually• Using unit tests

• Assemble pieces into more and more complete systems

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Function Stubs• Writing a larger programs, can't write everything at once• Outline!

def solve_climate_change(world_status):raise NotImplementedError

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Unit testing• "If it hasn't been tested, then it doesn't work"• Very hard to prove code works in every circumstance• Pick cases that are representative of all cases

• What inputs should be tested for this code?def compare(x, y):

"return True if x is at least 5 greater than y, False otherwise"

15

Actual test cases for PL questionsclass Test(unittest.TestCase):

@points(1)

@name("Testing with 'hi' and 'there'")

def test_one(self):r = concatenate_two_strings('hi',

'there')

self.assertEqual(r, 'hithere')

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