Debugging¶

Errors
- Syntax
- Exceptions
try/except

Debugging is the process of finding and fixing errors in a computer program.

Now that we’re writing functions, debugging will become really critical.

Clicker Question #1¶

Will I be able to define and execute this function?

def example_function(input_list):
    
    running_sum = 0
    for item in input_list:
        running_sum = running_sum + item
    
    special_value = input_list[3]
    
    return running_sum + special_value

A) Yes
B) No
C) Depends on the input_list
D) There’s no way to tell
E) I don’t know

### executing the function
example_function(['a','b','c','d'])

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-86157b94336b> in <module>()
      1 ### executing the function
----> 2 example_function(['a','b','c','d'])

<ipython-input-1-a4fb5262d9d1> in example_function(input_list)
      3     running_sum = 0
      4     for item in input_list:
----> 5         running_sum = running_sum + item
      6 
      7     special_value = input_list[3]

TypeError: unsupported operand type(s) for +: 'int' and 'str'

Errors¶

Errors are problems with code definition or execution that interrupt running Python code.

Syntax Errors¶

Syntax Errors
Indentation Errors

Syntax & Indentation Errors reflect code that doesn't follow Python structure, and will necessarily fail.

Syntax Error Examples¶

# will produce a syntax error
if True
    print('Yep.')

  File "<ipython-input-5-a91490a26c37>", line 2
    if True
            ^
SyntaxError: invalid syntax

Python does its best to tell you:

what type of error it is
and where it thinks it occurred (^)

# will produce a syntax error
# and specifically an indentation error
my_list = [1, 2]
for value in my_list:
print(value)

  File "<ipython-input-6-45596487aa45>", line 5
    print(value)
        ^
IndentationError: expected an indented block

Python gives you a readout about what it was expecting and where you appear to have gone wrong.

Exceptions¶

Exceptions are errors that occur when a code is executed.

For these, there’s nothing wrong with the syntax or structure of the code, but in your specific case and how you’re trying to use it, Python says ‘no’.

ZeroDivisionError¶

ZeroDivisionError occurs when you try to divide by zero.

# produces ZeroDivisionError
1 / 0

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-7-79e900a17bd3> in <module>()
      1 # produces ZeroDivisionError
----> 2 1 / 0

ZeroDivisionError: division by zero

Python specifies:

the Exception and specific type / error
points you to where the error occurred

NameError¶

NameError occurs when you try to access a name that Python does not know.

# Define a variable
variable = 12

# If you typo a name, you will get a NameError
varaible

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-9-4acade15292f> in <module>()
      1 # If you typo a name, you will get a NameError
----> 2 varaible

NameError: name 'varaible' is not defined

While it’s annoying, it’s helpful that Python doesn’t just guess that you meant ‘variable’….because sometimes Python would guess wrong. It’s better for Python to just give us the error.

# You also get a name error if you try to use the wrong operator for assignment
new_variable == 1

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-10-4f05423dede6> in <module>()
      1 # You also get a name error if you try to use the wrong operator for assignment
----> 2 new_variable == 1

NameError: name 'new_variable' is not defined

IndexError¶

IndexError occurs when you try to access an index that doesn’t exist.

my_list = [1, 2, 3]
my_list[5]

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-11-2c2a83518cf2> in <module>()
      1 my_list = [1, 2, 3]
----> 2 my_list[5]

IndexError: list index out of range

# Relatedly, 'KeyError' occurs if you ask for a dictionary key that doesn't exist
my_dictionary = {'name1' : 1, 'name2' : 2}
my_dictionary['name3']

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-12-84a1c00e4833> in <module>()
      1 # Relatedly, 'KeyError' occurs if you ask for a dictionary key that doesn't exist
      2 my_dictionary = {'name1' : 1, 'name2' : 2}
----> 3 my_dictionary['name3']

KeyError: 'name3'

ValueError¶

ValueError occurs when you try to use an illegal value for something.

int('cat')

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-13-1c6b28888bbe> in <module>()
----> 1 int('cat')

ValueError: invalid literal for int() with base 10: 'cat'

my_list = [1, 2, 3]
my_list.remove(0)

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-14-f00634ed638b> in <module>()
      1 my_list = [1, 2, 3]
----> 2 my_list.remove(0)

ValueError: list.remove(x): x not in list

TypeError¶

'a_string' + 12

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-15-57c293ee5895> in <module>()
----> 1 'a_string' + 12

TypeError: can only concatenate str (not "int") to str

Error Recap¶

Syntax Errors
- SyntaxError
- IndentationError
Exceptions
- ZeroDivisionError
- NameError
- Index Errors
  - IndexError
  - KeyError
- ValueError
- TypeError

Clicker Question #2¶

What type of error will the following code produce?

int('six')

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-16-196d25768c28> in <module>()
----> 1 int('six')

ValueError: invalid literal for int() with base 10: 'six'

A) Syntax
B) Name
C) Type
D) Index
E) Value

Clicker Question #3¶

What type of error will the following code produce?

if num > 0
    print("Greater than 0")

  File "<ipython-input-19-ad228ac1793f>", line 1
    if num > 0
              ^
SyntaxError: invalid syntax

A) Syntax
B) Name
C) Type
D) Index
E) Value

Clicker Question #4¶

What type of error will the following code produce?

if num > 0:
    print("Greater than 0")

A) Syntax
B) Name
C) Type
D) Index
E) Value

Stack Trace¶

The trace (log) of what Python did as it went through your code. Gets printed out if Python runs into an error.

running_sum = 0
my_list = [1, 2, 3, 4, 5]

for val in my_list:
    
    if val % 2 == 0:
        temp = val / (val - 4)
        #+= allows you to add the value on the right to the variable on the left 
        # and assign it to the variable on the left
        running_sum += temp 
        # equivalent to:
        # running_sum = running_sum + temp

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-20-215726ba149c> in <module>()
      5 
      6     if val % 2 == 0:
----> 7         temp = val / (val - 4)
      8         #+= allows you to add the value on the right to the variable on the left
      9         # and assign it to the variable on the left

ZeroDivisionError: division by zero

Sometimes these get really complex. We’re here to get better at interpreting these traces.

Note that if external functions are being used, these will get longer.

Try / Except¶

Exceptions do not necessarily have to lead to breaking the program - they can be programmatically dealt with, using 'try' and 'except'.

Try / Except Block¶

# Try / Except Block
try:
    # Tries to do this code
    pass # pass just says is not an operation; carry on
except:
    # If there is an error (an exception), keep going and do this instead
    pass

Try / Except Example¶

# Example: we want to get an input number from the user

my_num = input("Please type a number: ")

print('\nmy_num is: ', my_num)

Please type a number: asdlfjk

my_num is:  asdlfjk

Example with Try / Except¶

try:
    int(input('Number'))
except:
    print("nahhh")

Number7

Try / Except within a While Loop¶

input("text")

textasdlfkj

'asdlfkj'

ask_for_num = True

while ask_for_num:
    try:
        my_num = int(input("Please enter a number: "))
        ask_for_num = False
    except ValueError:
        print("Oops!  That was no valid number. Try again!")
        
print('\nmy_num is: ', my_num)

Please enter a number: asdfasdf;lkj
Oops!  That was no valid number. Try again!
Please enter a number: 5

my_num is:  5

More Try / Except¶

# define a function divide
def divide(num1, num2):
    return num1 / num2

print(divide(2, 0))

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-27-0cdf54a076ec> in <module>()
----> 1 print(divide(2, 0))

<ipython-input-26-62d4f4a69b89> in divide(num1, num2)
      1 # define a function divide
      2 def divide(num1, num2):
----> 3     return num1 / num2

ZeroDivisionError: division by zero

# define a function safe_divide
def safe_divide(num1, num2):
    
    try:
        output = num1 / num2
    except ZeroDivisionError:
        output = None
    
    return output

print(safe_divide(2, 0))

None

Raising Errors¶

You can also write code to raise an Exception if something unexpected happens.

Raise Exception Examples¶

raise is a keyword that tells Python you want to create your own error.

my_int = input('An integer please: ')
if not my_int.isnumeric():
    raise ValueError('I wanted a number! :(')
    
print('My integer is: ', my_int) 

An integer please: asdf;ajsdf

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-30-df6803c14de3> in <module>()
      1 my_int = input('An integer please: ')
      2 if not my_int.isnumeric():
----> 3     raise ValueError('I wanted a number! :(')
      4 
      5 print('My integer is: ', my_int)

ValueError: I wanted a number! :(

Clicker Question #5¶

Edit the code below (replacing --- with either values or variable names) so that when executed, this cell returns None.

num1 = 7
num2 = 0

try:
    output = num1 / num2
except ZeroDivisionError:
    output = None
    
print(output)

None

A) I did it!
B) I think I did it…
C) I’m totally lost.

Introductory Python

Debugging

Contents

Debugging¶

Clicker Question #1¶

Errors¶

Syntax Errors¶

Syntax Error Examples¶

Exceptions¶

ZeroDivisionError¶

NameError¶

IndexError¶

ValueError¶

TypeError¶

Error Recap¶

Clicker Question #2¶

Clicker Question #3¶

Clicker Question #4¶

Stack Trace¶

Try / Except¶

Try / Except Block¶

Try / Except Example¶

Example with Try / Except¶

Try / Except within a While Loop¶

More Try / Except¶

Raising Errors¶

Raise Exception Examples¶

Clicker Question #5¶