Different kinds of errors

Remember that we have different types of errors

• Syntax errors: code inconsistent with the Python language, the code will not run (i.e. not start).
• Runtime errors: exceptions occurring while the program runs, the code will crash.
• Semantic errors: the code does not what was intended

Try to make the following files error free:

• lecture_11_ex_errors_books.py
• lecture_11_ex_errors_runtime.py

Example: integer division

Remember the function for division:

def div(number, divisor):
    """ divides n/m and returns the quotient and rest """
    quotient = number // divisor
    remainder = number % divisor

    return quotient, remainder

print("Division of n/m:")
n = int(input("\tn = "))
m = int(input("\tm = "))
quotient, remainder = div(n, m)
print(f"Dividing {n} by {m} = {quotient}, rest = {remainder}")

User input and runtime errors

• User input can be anything \(\rightarrow\) runtime errors / crashes
  • runtime errors
  • undefined behavior: no crash but wrong results

# input function returns a string
# n_str = input("n = ")
n_str = "5.6"
n = int(n_str)
print(f"Provided number n = {n} with type = {type(n)}")

ValueError: invalid literal for int() with base 10: '5.6'

User input and runtime errors

• User input can be anything \(\rightarrow\) runtime errors / crashes
  • runtime errors
  • undefined behavior: no crash but wrong results

# input function returns a string
# n_str = input("n = ")
n_str = "4,1"
n = int(n_str)
print(f"Provided number n = {n} with type = {type(n)}")

ValueError: invalid literal for int() with base 10: '4,1'

User input and runtime errors

• User input can be anything \(\rightarrow\) runtime errors / crashes
  • runtime errors
  • undefined behavior: no crash but wrong results

# input function returns a string
# n_str = input("n = ")
n_str = "4.00"
n = int(n_str)
print(f"Provided number n = {n} with type = {type(n)}")

ValueError: invalid literal for int() with base 10: '4.00'

User input and runtime errors

• User input can be anything \(\rightarrow\) runtime errors / crashes
  • runtime errors
  • undefined behavior: no crash but wrong results

# input function returns a string
# n_str = input("n = ")
n_str = "0x002A"
n = int(n_str)
print(f"Provided number n = {n} with type = {type(n)}")

ValueError: invalid literal for int() with base 10: '0x002A'

User input and runtime errors

• User input can be anything \(\rightarrow\) runtime errors / crashes
  • runtime errors
  • undefined behavior: no crash but wrong results

# input function returns a string
# n_str = input("n = ")
n_str = "010"
n = int(n_str)
print(f"Provided number n = {n} with type = {type(n)}")

Provided number n = 10 with type = <class 'int'>

User input and runtime errors

• User input can be anything \(\rightarrow\) runtime errors / crashes
  • runtime errors
  • undefined behavior: no crash but wrong results

# input function returns a string
# n_str = input("n = ")
n_str = "5-2"
n = int(n_str)
print(f"Provided number n = {n} with type = {type(n)}")

ValueError: invalid literal for int() with base 10: '5-2'

User input and runtime errors

• User input can be anything \(\rightarrow\) runtime errors / crashes
  • runtime errors
  • undefined behavior: no crash but wrong results

print(f"The division {-9} / {4} = {-9 // 4} with remainder = {-9 % 4}")
print(f"The division {9} / {-4} = {9 // -4} with remainder = {9 % -4}")

The division -9 / 4 = -3 with remainder = 3
The division 9 / -4 = -3 with remainder = -3

User input and runtime errors

• Two options to prevent runtime error crashes:
  1. Anticipate any error: Validate the input data
  2. Handle the error when it occurs

Option (1): test input on validity

• We can use eval function to evaluate a string expression
• function isinstance(object, type) checks if an object is of type

# n_str = input("provide a number for n = ")
n_str = "1.3"
n = eval(n_str)
print(f"Provided number n = {n} with type = {type(n)}")
# return validity
if not isinstance(n, int):
  print("Input is invalid !")

Provided number n = 1.3 with type = <class 'float'>
Input is invalid !

Option (2): error handling

• When a runtime error occurs Python raises an exception
• To catch an error we use the try - except block
  • Statements in the try block are executed until an exception is encountered
  • The except block is executed on encountering an exception

try:
  <statements>
except:
  <statements>

Option (2): error handling

• When a runtime error occurs Python raises an exception
• To catch an error we use the try - except block
  • Statements in the try block are executed until an exception is encountered
  • The except block is executed on encountering an exception

# n_str = input("provide a number for n = ")
try:
  n_str = "1.3"
  n = int(n_str)
except:
  print("Input is invalid !")

Input is invalid !

Recovering from runtime errors

• If the user input was invalid:
  • Allow the user to try again
  • Specify the problem to the user
• Look at the Python script file: lecture_11_ex_errors_validate_input.py

More complex error-handling

try:
  n = int("3"); m = int("5.5")
  quotient = n // m
except ZeroDivisionError:
  print("Can't divide by zero")
except ValueError:
  print("Please provide two integers")
else:
  print(f"quotient = {quotient}")
finally:
  # Always executed
  print(f"You tried to divide n / m")

Please provide two integers
You tried to divide n / m

More complex error-handling

try:
  n = int("5"); m = int("0")
  quotient = n // m
except ZeroDivisionError:
  print("Can't divide by zero")
except ValueError:
  print("Please provide two integers")
else:
  print(f"quotient = {quotient}")
finally:
  # Always executed
  print(f"You tried to divide n / m")

Can't divide by zero
You tried to divide n / m

More complex error-handling

try:
  n = int("6"); m = int("2")
  quotient = n // m
except ZeroDivisionError:
  print("Can't divide by zero")
except ValueError:
  print("Please provide two integers")
else:
  print(f"quotient = {quotient}")
finally:
  # Always executed
  print(f"You tried to divide n / m")

quotient = 3
You tried to divide n / m

More complex error-handling: else

• else keyword allows to split the error handling of the try block:
  • try-part which you want to catch errors now
  • else-part which you have code that has its own error handling, or should crash if a problem occurs.

More complex error-handling: finally

• Usage of finally is used to gracefully crash, examples would be:
  • you opened a file, a problem occurred but it needs to be closed before stopping
  • script saved temporary results: clean up

Input/Output with Python (I/O)

• Command line arguments
• We can read and writes files to and from the hard disk
  • text files
  • formatted text: xml, html, markdown, postscript
  • multimedia: music, images, video

Input/Output with Python (I/O)

• Command line arguments
• We can read and writes files to and from the hard disk
  • text files
  • formatted text: xml, html, markdown, postscript
  • multimedia: music, images, video
• Interactive input events:
  • keyboard and mouse
  • we have to constantly “wait” for them? How?
• Graphical user interfaces

Executing a script in the terminal or command line

• Command line arguments can be found with sys.argv
• Code to print the arguments:

import sys
for i, arg in enumerate(sys.argv):
  print(f"Argument {i} = {sys.argv[i]}")

• Look at the Python script file: lecture_11_ex_command_line.py

Executing a script in the terminal or command line

• More advanced argument handling using argparse
  • define type of argument
  • help description
  • positional vs. named arguments
• Look at the Python script file:
  lecture_11_ex_argparse.py

Reading files from the hard disk

• Make sure the file path exists
• Be careful with file path separators
  • Unix/Linux uses / symbols (forward slash)
  • Windows uses \ symbols (backslash)
  • The \ separator should be escaped \\

# Problem with file paths in Windows:
file_path_in_windows = "C:\Users\mich\Documents\my_file.txt"
file_path_in_linux = "/home/mich/Documents/my_file.txt"

Python expects / or \\ as file separator !

Reading files from the hard disk

• Make sure the file path exists
• Be careful with file path separators
  • Unix/Linux uses / symbols (forward slash)
  • Windows uses \ symbols (backslash)
  • The \ separator should be escaped \\
• While you occupy a file for reading/adapting it might not be accessible by others
  • Important to close the file after finishing
  • What happens if your script crashes while it was writing to a file?

Text files

• Opening the file and closing after done
• When do you use this method:
  • Multiple separate read/write access
  • for a longer time, doing complex actions

file_path = "samples/sample_text_file.txt"
file = open(file_path)
print(file.read())
file.close()

This is the first line of the file.
The second line ...
I like banana cake !

Text files

• Opening using the with keyword
• Automatically closes the file after it

file_path = "samples/sample_text_file.txt"
with open(file_path) as file:
  print(file.read())

This is the first line of the file.
The second line ...
I like banana cake !

Handling files that don’t exist

• Opening using the open keyword
• Ensure that file is closed afterwards by finally

file_path = "samples/sample_text3_file.txt"
try:
  file = open(file_path, "r")
  print(file.read())
except FileNotFoundError:
  print("The file does not exist")
except:
  print("Some issue occurred during file reading")
finally:
  file.close()

The file does not exist

Vector graphics: svg-files

• Open the file in PyCharm

file_path = "samples/sample_vector_graphics.svg"
with open(file_path) as file:
  print(file.read())

<svg version="1.1"
     width="400" height="400"
     xmlns="http://www.w3.org/2000/svg">

  <rect width="100%" height="100%" fill="red" />
  <circle cx="150" cy="100" r="80" fill="green" />
  <text x="150" y="125" font-size="60" text-anchor="middle" fill="white">SVG</text>

</svg>

Keyboard and mouse input

• Install the package pynput
• Look up the documentation to control keyboard and mouse, example:

from pynput.mouse import Button, Controller

mouse = Controller()

# Read pointer position
print(f"Mouse position is {mouse.position}")

# Set pointer position
mouse.position = (100, 200)
print(f"Mouse moved to {mouse.position}")