Common Python exceptions#
Python has dozens of specific errors that can be raised when code fails to run. Below are a few common ones that you may encounter in Activity 3.
TypeError#
Occurs when an operation is applied to an object of an inappropriate type.
# Example: Trying to add a number and a string
1 + 'string' # This will raise a TypeError
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Cell In[1], line 2
1 # Example: Trying to add a number and a string
----> 2 1 + 'string' # This will raise a TypeError
TypeError: unsupported operand type(s) for +: 'int' and 'str'
ValueError#
Raised when a function receives an argument of the right type but an invalid value.
Example:
int('abc')(trying to convert an invalid string to an integer).
int("abc")
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
Cell In[2], line 1
----> 1 int("abc")
ValueError: invalid literal for int() with base 10: 'abc'
KeyError#
Raised when a dictionary key is not found.
Example:
my_dict['nonexistent_key'](trying to access a key that doesn’t exist in the dictionary).
# Example: Accessing a nonexistent key in a dictionary
my_dict = {"a": 1, "b": 2}
my_dict['nonexistent_key']
---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
Cell In[3], line 3
1 # Example: Accessing a nonexistent key in a dictionary
2 my_dict = {"a": 1, "b": 2}
----> 3 my_dict['nonexistent_key']
KeyError: 'nonexistent_key'
IndexError#
Raised when an invalid index is used to access a list or tuple.
Example:
my_list[10](trying to access the 11th element of a list with fewer elements).
my_list = [1, 2, 3]
my_list[10]
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
Cell In[4], line 2
1 my_list = [1, 2, 3]
----> 2 my_list[10]
IndexError: list index out of range
AttributeError#
Raised when an object does not have a specific attribute or method.
my_string = "Hello"
my_string.nonexistent_method()
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
Cell In[5], line 2
1 my_string = "Hello"
----> 2 my_string.nonexistent_method()
AttributeError: 'str' object has no attribute 'nonexistent_method'
FileNotFoundError#
A FileNotFoundError occurs in Python when the code attempts to open or access a file that does not exist at the specified path.
with open("data/nonexistent_file.json", "r") as file:
data = file.read()
---------------------------------------------------------------------------
FileNotFoundError Traceback (most recent call last)
Cell In[6], line 1
----> 1 with open("data/nonexistent_file.json", "r") as file:
2 data = file.read()
File ~/work/lessons/lessons/.nox/docs-test/lib/python3.11/site-packages/IPython/core/interactiveshell.py:343, in _modified_open(file, *args, **kwargs)
336 if file in {0, 1, 2}:
337 raise ValueError(
338 f"IPython won't let you open fd={file} by default "
339 "as it is likely to crash IPython. If you know what you are doing, "
340 "you can use builtins' open."
341 )
--> 343 return io_open(file, *args, **kwargs)
FileNotFoundError: [Errno 2] No such file or directory: 'data/nonexistent_file.json'
By catching this exception, you can
Raise a kinder and more informative message.
Direct the user toward the next steps
FUTURE: write tests for this step of the workflow (if you are creating a package!) that make sure that it handles a bad file path properly.
from pathlib import Path
file_path = Path("data") / "nonexistent_file.json"
try:
with open(file_path, "r") as file:
data = file.read()
except FileNotFoundError as fe:
raise FileNotFoundError(f"Oops! it looks like you provided a path to a file that doesn't exist. You provided: {file_path}. Make sure the file path exists. ")
---------------------------------------------------------------------------
FileNotFoundError Traceback (most recent call last)
Cell In[7], line 5
4 try:
----> 5 with open(file_path, "r") as file:
6 data = file.read()
File ~/work/lessons/lessons/.nox/docs-test/lib/python3.11/site-packages/IPython/core/interactiveshell.py:343, in _modified_open(file, *args, **kwargs)
337 raise ValueError(
338 f"IPython won't let you open fd={file} by default "
339 "as it is likely to crash IPython. If you know what you are doing, "
340 "you can use builtins' open."
341 )
--> 343 return io_open(file, *args, **kwargs)
FileNotFoundError: [Errno 2] No such file or directory: 'data/nonexistent_file.json'
During handling of the above exception, another exception occurred:
FileNotFoundError Traceback (most recent call last)
Cell In[7], line 8
6 data = file.read()
7 except FileNotFoundError as fe:
----> 8 raise FileNotFoundError(f"Oops! it looks like you provided a path to a file that doesn't exist. You provided: {file_path}. Make sure the file path exists. ")
FileNotFoundError: Oops! it looks like you provided a path to a file that doesn't exist. You provided: data/nonexistent_file.json. Make sure the file path exists.
If you don’t raise the error but instead provide a print statement, you can provide a simple, clean output without the full “stack” or set of Python messages that provides the full “tracking” or traceback of where the error originated.
The challenge with not raising a FileNotFound error is that it will be a bit trickier to test the output.
file_path = Path("data") / "nonexistent_file.json"
try:
with open(file_path, "r") as file:
data = file.read()
except FileNotFoundError as fe:
print(f"Oops! it looks like you provided a path to a file that doesn't exist. You provided: {file_path}. Make sure the file path exists. ")
Oops! it looks like you provided a path to a file that doesn't exist. You provided: data/nonexistent_file.json. Make sure the file path exists.