Exception handling tutorial¶

Imports¶

In [1]:

from sys import path
from os.path import abspath

# Tell python to search for the files and modules starting from the working directory
module_path = abspath("..")
if module_path not in path:
    path.append(module_path)

from sys import path from os.path import abspath # Tell python to search for the files and modules starting from the working directory module_path = abspath("..") if module_path not in path: path.append(module_path)

Note: If we don't want to be telling Python where to search for our library, we can install it (in editable mode) in our conda environment using

pip install -e .

In [2]:

from mypackage import Vector

from mypackage import Vector

Custom exceptions¶

Exception errors occur whenever syntactically correct Python code results in an error. In our Vector class, we raised an error whenever the norm of the initialized vector was greater than the module constant MAX_NORM. This custom error class, NormError, was defined as a subclass of the built-in exception class ValueError.

In [3]:

vector = Vector(100, 200)

vector = Vector(100, 200)

---------------------------------------------------------------------------
NormError                                 Traceback (most recent call last)
Cell In[3], line 1
----> 1 vector = Vector(100, 200)

File ~/Documents/Code projects/Python-tips-tools/mypackage/vector/vector.py:70, in Vector.__init__(self, x, y)
     67 self.y: float = y
     69 if self.norm > MAX_NORM:
---> 70     raise NormError(self.norm)

NormError: Norm = 223.60679774997897, but it cannot be greater than 100.

Exception handling¶

Custom exceptions come in handy when handling errors. For example, we can create a vector given two components and, in case the norm is too big, handle the NormError to create a new "renormalized" vector

In [5]:

from mypackage import NormError

x = 100
y = 200

try:
    vector = Vector(x, y)
    print(f"{vector = !s}")

except NormError:
    print("Norm is too big, resizing the vector...")
    x = x % 71
    y = y % 71
    vector = Vector(x, y)
    print(f"{vector = !s}")

finally:
    print("\nThe try-except statement ended!")

from mypackage import NormError x = 100 y = 200 try: vector = Vector(x, y) print(f"{vector = !s}") except NormError: print("Norm is too big, resizing the vector...") x = x % 71 y = y % 71 vector = Vector(x, y) print(f"{vector = !s}") finally: print("\nThe try-except statement ended!")

Norm is too big, resizing the vector...
vector = (29, 58)

The try-except statement ended!