====== Computer Lab 09, debugging, code testing, exceptions ======

Error types:
  - **Syntax error** (omitting colon at the end of the ''def'' statement, unclosed bracket, non-matching quotation marks, ...)
  - **Runtime error** (division by zero, indexing error, using Python's keyword for a variable name, ...)
  - **Semantic error** (error in the program's logic, hard to spot, interpreter nor runtime system is of no help here)


==== Syntax Error ====
<code python>
def sum_numbers(lst)
    sum = 0
    for item in lst:
        sum += item
    return sum
</code>
<code>
  File "syntax01.py", line 1
    def sum_numbers(lst)   
                       ^   
SyntaxError: invalid syntax
</code>


----

<code python>
def add_gold(inv, inc_gold):
    inv["gold"] += inc_gold

if __name__ == "__main__":
    inventory = {
    'gold' : 500,
    'pouch' : ['flint', 'twine', 'gemstone'],
    'backpack' : ['xylophone','dagger', 'bedroll','bread loaf']
    
    
    add_gold(inventory, 50)
    print(inventory)
</code>
<code>
  File "syntax02.py", line 11
    add_gold(inventory, 50)
           ^
SyntaxError: invalid syntax
</code>

----

<code python>
mytuple = ("apple", "banana', "cherry")
 
for x in mytuple:
  print(x)
</code>
<code>
  File "syntax03.py", line 1
    mytuple = ("apple", "banana', "cherry")
                                        ^
SyntaxError: invalid syntax
</code>

==== Runtime Error ====
  * When a runtime error occurs an **exception object** is created.
  * Then, traceback and an error message are printed out.
  * If not handled, the program force ends.

<WRAP group>
<WRAP half column>
**NAME ERROR**
<code python>
if __name__ == "__main__":
    value += 1
    print("Value = ", value)
</code>
<code>
Traceback (most recent call last):
  File "runtime01.py", line 2, in <module>
    value += 1
NameError: name 'value' is not defined
</code>
</WRAP>

<WRAP half column>
**TYPE ERROR (wrong number of parameters)**
<code python>
def sum_numbers(a, b):
    return a + b

if __name__ == "__main__":
    nr_1 = 8
    nr_2 = 10
    nr_3 = -6
    sum = sum_numbers(nr_1, nr_2, nr_3)
    print("sum = ", sum)
</code>
<code>Traceback (most recent call last):
  File "runtime02.py", line 8, in <module>
    sum = sum_numbers(nr_1, nr_2, nr_3)
TypeError: sum_numbers() takes 2 positional arguments but 3 were given
</code>
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**TYPE ERROR (division of strings)**
<code python>
def div_numbers(a, b):
    return a / b

if __name__ == "__main__":
    nr_1 = input("Enter first number: ")
    nr_2 = input("Enter second number: ")
    sum = div_numbers(nr_1, nr_2)
    print("Quotient = " + str(sum))
</code>
<code>
Enter first number: 10
Enter second number: 5
Traceback (most recent call last):
  File "runtime02.py", line 7, in <module>
    sum = div_numbers(nr_1, nr_2)
  File "runtime02.py", line 2, in div_numbers
    return a / b
TypeError: unsupported operand type(s) for /: 'str' and 'str'
</code>
</WRAP>

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**TYPE ERROR (trying to concatenate //str// and //int//)**
<code python>
def div_numbers(a, b):
    return a / b

if __name__ == "__main__":
    nr_1 = float(input("Enter first number: "))
    nr_2 = float(input("Enter second number: "))
    sum = div_numbers(nr_1, nr_2)
    print("Quotient = " + sum)
</code>
<code>
Enter first number: 10
Enter second number: 5
Traceback (most recent call last):
  File "runtime02.py", line 8, in <module>
    print("Quotient = " + sum)
TypeError: must be str, not float
</code>
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<WRAP half column>
**KEY ERROR (indexing dictionary with a non-existent key)**
<code python>
fridge = {"apple": 10, 
            "banana": 8,
            "cherry": 50}
 
print(fridge["kiwi"])
</code>
<code>
Traceback (most recent call last):
  File "runtime03.py", line 5, in <module>
    print(fridge["kiwi"])
KeyError: 'kiwi'
</code>
</WRAP>

<WRAP half column>
**KEY ERROR (indexing dictionary with a wrong data type)**
<code python>
fridge = {"apple": 10, 
            "banana": 8,
            "cherry": 50}
 
print(fridge[0])
</code>
<code>
Traceback (most recent call last):
  File "runtime03.py", line 5, in <module>
    print(fridge[0])
KeyError: 0
</code>
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**INDEX ERROR (how to make this //pythonic//?)**
<code python>
programming_languages = ["Java", "Python", "C++"]
count = 0 

while count <= len(programming_languages):
	print(programming_languages[count])
	count += 1
</code>
<code>
Java
Python
C++
Traceback (most recent call last):
  File "syntax04.py", line 5, in <module>
    print(programming_languages[count])
IndexError: list index out of range
</code>
</WRAP>

<WRAP half column>
**ZERO DIVISION ERROR**
<code python>
import random 

def get_inverted(x):
    return 1/x

if __name__ == "__main__":
    random.seed(128)
    for _ in range(10):
        value = random.randrange(0,10)
        inv_value = get_inverted(value)
        print("1/{} is {}.".format(value, inv_value))
</code>
<code>
1/3 is 0.3333333333333333.
1/6 is 0.16666666666666666.
1/8 is 0.125.
1/5 is 0.2.
1/2 is 0.5.
1/7 is 0.14285714285714285.
Traceback (most recent call last):
  File "08_runtime.py", line 10, in <module>
    inv_value = get_inverted(value)
  File "08_runtime.py", line 4, in get_inverted
    return 1/x
ZeroDivisionError: division by zero
</code>
</WRAP>
</WRAP>

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<WRAP half column>
**FILE NOT FOUND ERROR**
<code python>
filename = "dummyfile.txt"
f = open(filename, "r")
lines =  f.readlines()
for line in lines:
    print(line)
f.close()
</code>
<code>
Traceback (most recent call last):
  File "runtime05.py", line 2, in <module>
    f = open(filename, "r")
FileNotFoundError: [Errno 2] No such file or directory: 'dummyfile.txt'
</code>
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=== Handling Runtime Errors ===


  * If not handled, the program stops after the exception is identified and an error message is printed out.
  * Usually, the parts of the code which might cause an exception are easily identified. 
  * Use try-except to catch the exception and handle this situation yourself without stopping the program.

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**FLAWED VERSION**
<code python>
def div_numbers(a, b):
    return a / b

if __name__ == "__main__":
    nr_1 = input("Enter first number: ")
    nr_2 = input("Enter second number: ")
    sum = div_numbers(nr_1, nr_2)
    print("Quotient = " + str(sum))
</code>
<code>
Enter first number: 10
Enter second number: 5
Traceback (most recent call last):
  File "runtime02.py", line 7, in <module>
    sum = div_numbers(nr_1, nr_2)
  File "runtime02.py", line 2, in div_numbers
    return a / b
TypeError: unsupported operand type(s) for /: 'str' and 'str'
</code>
</WRAP>

<WRAP half column>
**ATTEMPT #1: TRY-EXCEPT in the function**
<code python>
def div_numbers(a, b):
    try: 
        q = a / b
        return q
    except TypeError:
        return None
 
if __name__ == "__main__":
    nr_1 = input("Enter first number: ")
    nr_2 = input("Enter second number: ")
    sum = div_numbers(nr_1, nr_2)
    
    if sum is not None:
        print("Quotient = " + str(sum))
    else:
        print("Error when calling the div_numbers() function.")
</code>
<code>
Enter first number: 10
Enter second number: 5
Error when calling the div_numbers() function.
</code>
</WRAP>
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**ATTEMPT #2: TRY-CATCH in the function + print(e)**
<code python>
try: 
        q = a / b
        return q
except TypeError as e:
        print(e)
        return None
</code>
<code>
TypeError: unsupported operand type(s) for /: 'str' and 'str'
</code>
</WRAP>

<WRAP half column>
**ATTEMPT #3: TRY-CATCH in the function + print traceback**
<code python>
import traceback 

def div_numbers(a, b):
    try: 
        q = a / b
        return q
    except TypeError:
        # printing stack trace 
        traceback.print_exc() 
        return None
</code>
<code>
Traceback (most recent call last):
  File "handling01.py", line 5, in div_numbers
    q = a / b
</code>
</WRAP>
</WRAP>

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<WRAP half column>
**ATTEMPT #4: SANITIZING INPUT DATA**
<code python>
def div_numbers(a, b):
        q = a / b
        return q
 
if __name__ == "__main__":
    try:
        nr_1 = float(input("Enter first number: "))
        nr_2 = float(input("Enter second number: "))

        sum = div_numbers(nr_1, nr_2)
        print("Quotient = " + str(sum))
    except ValueError:
        print("You did not enter valid numbers!")        
</code>
</WRAP>

<WRAP half column>
**ATTEMPT #5: RAISING EXCEPTION**
<code python>
def div_numbers(a, b):
    if type(a) is not float or type(b) is not float:
        raise TypeError("a or b is not float!")

    q = a / b
    return q
 
if __name__ == "__main__":

    nr_1 = "a"
    nr_2 = 5.0

    try:
        sum = div_numbers(nr_1, nr_2)
        print("Quotient = " + str(sum))
    except TypeError as e:
        print(e)
</code>
<code>
a or b is not float!
</code>

</WRAP>
</WRAP>

==== Semantic Error ====

  * Usually very difficult to spot!
  * Mismatch between the problem specification and what the code actually does. 
  * Is there something the program was supposed to do but which doesn't seem to be happening?
  * Is something happening that shouldn't?
  * Several methods to find where the problem is:
  - Create a set of well-defined inputs and corresponding outputs. Compare these ground-truth outputs with the actual outputs of your code. Based on this, try to identify the place where the error could originate. 
  - Place diagnostic print() statements in your code to inspect the content of your variables.
  - Use the debugger to step through your program run by line. Set up (conditional) breakpoints. 
  - Write ''doctests'' (you already know) + ''unittest'' (will be explained once you learn about objects) to make ensure that the code does not break with changes.

==== Spot the Error ====
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**EXERCISE 1**
<code python>
def sum_even_numbers(lst):
    sum = 0
    for item in lst:
        if item % 2 == 0:
            sum += item
        return sum

if __name__ == "__main__":
    input_list = [9, 1, 0, 2, 8]
    sum_of_list = sum_even_numbers(input_list)
    print(sum_of_list)
</code>
</WRAP>

<WRAP half column>
**EXERCISE 2**
<code python>
x = float(input('Enter a number: '))
y = float(input('Enter a number: '))

z = x+y/2
print ('The average of the two numbers you have entered is:',z)
</code>
<code>
Enter a number: 3
Enter a number: 4
The average of the two numbers you have entered is: 5.0
</code>
</WRAP>
</WRAP>

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<WRAP half column>
**EXERCISE 3**
<code python>
# A function to append a list onto itself, with the intention of
# returning a new list, but leaving the input unaltered
def double_list(in_list):
    """Append a list to itself."""
    in_list += in_list
    return in_list

# Make a list
my_list = [3, 2, 1]

# Double it
my_list_double = double_list(my_list)

# Later on in our program, we want a sorted my_list
my_list.sort()

# Let's look at my_list:
print('We expect [1, 2, 3]')
print('We get   ', my_list)
</code>
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=== Weekly homework 09 - Piecewise function ===
In this homework, you will need to write 2 functions in one module (file):
  * Function ''my_pwc_function'' implementing a piecewise function defined by the following formula:
\[ f(x)= \begin{cases} 
      (x-6)^2+5 & 6 \le x \\
      5 & 4 \leq x\lt 6 \\
      \frac{5}{2}x-5 & 2 \leq x\lt 4 \\
      \text{undefined} & x \lt 2 
   \end{cases}
\]
<code python>
def my_pwc_function(x):
    """Returns a function value of the piecewise function specified in the problem setting
 
    :param x: float, parameter of the mathematical function
    :return: float, corresponding function value 
    :raises: ValueError: for such 'x' where the piecewise function is undefined
    
    >>> my_pwc_function(5)
    5.0
    >>> my_pwc_function(3)
    2.5
    """
</code>

  * Function ''get_function_values(par_lst)'' which computes function values (using ''my_pwc_function()'') of all numbers given in the ''par_lst'' list and stores all the results in a dictionary. In case of an undefined function value, place 'NA' string as a value to the corresponding key.
<code python>
def get_function_values(par_lst):
    """Returns dictionary where the key are numbers from par_list and values are corresponding
    function values.
 
    :param par_lst: list of floats, values to be passed to the piecewise function
    :return: dict of x:my_pwc_function(x), function values
   
    >>> get_function_values([5, 1, 3])
    {5:5.0, 1:'NA', 3:2.5}

    """
</code>
Required filename: ''09_weekly_hw.py''.
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