Errors and exceptions - Learn Python with BNHR

Mistakes are inevitable in data work. Exception handling allows you to catch them.

Errors¶

issues/mistakes in a program that prevent it from executing successfully
common types include syntax errors, runtime errors, and logical errors

Exceptions¶

events that occur during the execution of a program that disrupts the normal flow of instructions
common examples include division by zero, accessing an index that does not exist, or trying to open a file that doesn’t exist

Common Exceptions Types:

ZeroDivisionError: Dividing by zero.
ValueError: Trying to convert text (“Five”) to a number.
IndexError: Trying to access item #5 in a list of only 3 items.

Handling exceptions¶

try, except, finally
- try: Contains the block of code where an exception might occur.
- except: Specifies the block of code to execute if an exception occurs in the try block.
- finally: Contains code that will be executed no matter what, whether an exception occurred or not.

x, y = 1, 0

print(x/y)

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
Cell In[1], line 3
      1 x, y = 1, 0
----> 3 print(x/y)

ZeroDivisionError: division by zero

x, y = 1, 0

try:
    print(x/y)
except:
    print("We couldn't do the division")

x, y = 1, 0

try:
    print(x/y)
except ZeroDivisionError as e:
    print("We couldn't do the division. ERROR: {}".format(e))

x, y = 1, 0

try:
    print(x/y)
    
except ZeroDivisionError as e:
    print("We couldn't do the division. Reason: {}".format(e))
finally:
    print("Maybe try again next time?")

# Example: Calculating Yield per Hectare (with risky math)
def calculate_yield(production, hectares):
    try:
        result = production / hectares
        return f"{result:.2f} tons/ha"
    except ZeroDivisionError:
        return "Error: Cannot divide by zero (Hectares is 0)"
    except TypeError:
        return "Error: Inputs must be numbers"

# Test cases
print(calculate_yield(100, 10))  # Normal
print(calculate_yield(100, 0))   # Error: Division by Zero
print(calculate_yield("100", 5)) # Error: Wrong Type

File and Exception Handling Combined¶

Real-world data is messy. A file might be missing, or a user might enter “N/A” in a numeric column. We combine with open and try-except to build robust pipelines.

target_file = "barangay_farms.txt"
# target_file = "missing_file.txt" # Uncomment this line to test the error

try:
    with open(target_file, "r") as f:
        header = next(f) # Skip the header line
        
        for line in f:
            parts = line.strip().split(",")
            farm_id = parts[0]
            crop = parts[1]
            
            try:
                # Try to convert area to float
                area = float(parts[2])
                print(f"Farm {farm_id} ({crop}): {area} ha")
            except ValueError:
                print(f"Warning: Invalid area for Farm {farm_id}")

except FileNotFoundError:
    print(f"Critical Error: The file '{target_file}' was not found.")

# target_file = "barangay_farms.txt"
target_file = "missing_file.txt" # Uncomment this line to test the error

try:
    with open(target_file, "r") as f:
        header = next(f) # Skip the header line
        
        for line in f:
            parts = line.strip().split(",")
            farm_id = parts[0]
            crop = parts[1]
            
            try:
                # Try to convert area to float
                area = float(parts[2])
                print(f"Farm {farm_id} ({crop}): {area} ha")
            except ValueError:
                print(f"Warning: Invalid area for Farm {farm_id}")

except FileNotFoundError:
    print(f"Critical Error: The file '{target_file}' was not found.")