Debugging and Error Handling in Python
- Explain common errors and how they occur in Python
- Use pdb to set breakpoint and step through code
- Use try and except blocks to handle errors
Common Types of Error in Python
You’re going to make mistakes…
So how do you fix them ?
Understand them!
Let’s start with some common errors!
SyntaxError
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Occurs when Python encounters incorrect syntax (something it doesn’t parse)
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Usually due to typos or not knowing Python well enough
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def first: # SyntaxError
None = 1 # SyntaxError
return # SyntaxError
NameError
This occur when a variable is not defined, i.e it hasn’t been assigned
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test
# NameError: name 'test' is not d efined
TypeError
Occurs when:
- An operation or function is applied to the wrong type
- Python cannot interpret an operation on two data types
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len(5)
# TypeError: object of type 'int' has no len()
"Abdoulaye" + []
# TypeError: must be str, not list
IndexError
Occurs when you try to access an element in a list using an invalid index(i.e one that is outside the range of the list or string):
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name = "Bah"
name[0] # B
name[3] # IndexError: string index out of range
nums = (1,2,3,4)
nums[6] # IndexError: tuple index out of range
list = ["Bonjour"]
list[2] # IndexError: list index out of range
ValueError
This occurs when a built-in operation or function receives an argument that has the right type but an inappropriate value:
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int("5") # 5 --> This is OK
int("houpla")
# ValueError: invalid literal for int() with base 10: 'houpla'
KeyError
This occur when a dictionary does not have a specific key:
Raising our Own Errors
Raise Your Own Exception!
In python we can also throw errors using the raise keyword. This is helpful when creating your own kinds of exception and error messages.
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raise ValueError('invalid value')
# ValueError: invalid value
raise NameError('Abdoulaye')
NameError: Abdoulaye
Exemple
Let’s define a function that takes a text and colorize it
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def colorize(text, color):
colors = ("cyan", "yellow", "blue", "green", "magenta")
if type(color) is not str:
raise TypeError("color must be intance of string")
if type(text) is not str:
raise TypeError("text must be instance of string")
if color not in colors:
raise ValueError("color is invalid color")
print(f"Printed {text} in {color}")
colorize("hello", "red") # ValueError: color is invalid color
colorize("hi", 10) # TypeError: color must be instance of string
colorize(34, "green") # TypeError: text must be isntance of string
Try and Except Blocks
Handle Errors!
In Python, it is strongly encouraged to use try/except blocks, to catch exceptions when we can do something about them. Let’s see what that looks like.
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d = {"name": "Bah"}
d["city"] # KeyError: 'city'
def get(d, key):
try:
return d[key]
except KeyError:
return None
print(get(d, "name")) # Bah
print(get(d, "city")) # None
Try Except Else and Finally
Capture user input as a number and throw erros if it’s not a number
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while True:
try:
num = int(input("please enter a number: "))
except:
print("That's not a number!")
else:
print("Good job, you entered a number!")
break
finally:
print("Runs no matter what!")
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def divide(a, b):
try:
result = a/b
except (ZeroDivisionError, TypeError) as err:
print("Something went wrong!")
print(err)
else:
print(f"{a} divided by {b} is {result})
Debugging with PDB
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def add_numbers(a, b, c, d):
# importing the python debugger
import pdb; pdb.set_trace()
return a + b + c + d
add_numbers(1,2,3,4)
Common PDB Commands
- l (list)
- n (next line)
- p (print)
- c (continue - finishes debugging)