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Common Errors

21. Common Errors#

This glossary provides a list of common errors with explanations and details on how to handle them.

IndexError#

Meaning: An IndexError occurs when you try to access an index in a sequence (like a list or a tuple) that is outside the valid range of indices for that sequence. This error indicates that you are attempting to access an index that is either negative or beyond the end of the sequence. Python sequences are zero-indexed, meaning the first element is at index 0, the second element at index 1, and so forth. Trying to access an index that doesn’t exist leads to this error.

How to Address:

  • Check Index Boundaries: Ensure that the index you’re using is within the valid range of the sequence. For example, if a list has 5 elements, valid indices are from 0 to 4.

  • Verify List Length: Use functions like len to check the length of the sequence before accessing an element by index.

  • Handle Negative Indices Carefully: Negative indices count from the end of the sequence, so make sure you’re not inadvertently using an index that is too negative.

KeyError#

Meaning: A KeyError occurs when you try to access a key in a dictionary that does not exist. In Python, dictionaries are collections of key-value pairs, and attempting to retrieve a value using a non-existent key triggers this error.

How to Address:

  • Check the Key Existence: Before accessing a key, use the in operator to check if the key exists in the dictionary. For example: if key in my_dict.

  • Use dict.get Method: Instead of directly accessing the key, use the get method, which returns None or a default value if the key is not found. Example: value = my_dict.get(key, default_value).

  • Exception Handling: Wrap the dictionary access in a try-except block to catch the KeyError and handle it appropriately. Example:

try:
    value = my_dict[key]
except KeyError:
    # Handle the error, maybe use a default value

SyntaxError#

Meaning: A SyntaxError occurs when the Python interpreter encounters code that doesn’t follow the correct structure or rules of the Python language. This could be due to missing punctuation (like a colon : or parentheses ()), incorrect indentation, or improperly used keywords. The interpreter is unable to parse the code because it doesn’t conform to Python’s syntax rules.

How to Address:

  • Carefully check the code where the error is reported. However, be aware that the actual mistake might be on the line above the one flagged by the interpreter. This is common if, for example, a line ends without closing parentheses or a colon is missing at the end of a statement that should precede an indented block.

  • Review the preceding lines for proper structure, ensuring that all brackets are closed and that keywords are correctly used.

  • Fix any indentation issues, as Python relies heavily on indentation to define code blocks.

TypeError#

Meaning: A TypeError occurs when an operation or function is applied to an object of inappropriate type. This typically happens when operations or functions receive arguments that are not of the expected type, or when you try to perform operations between incompatible types.

How to Address:

  • Check Data Types: Verify (using the type function or the debuger) that the variables or values you are working with are of the correct type required by the operation or function.

  • Convert Types: Use type conversion functions to ensure that data types match what is expected (e.g., converting strings to integers if needed).

  • Review Function Definitions: Make sure that you are passing the correct types of arguments to functions or methods.

  • Examine Operator Use: Ensure that the operators you are using are compatible with the data types of the operands.

ValueError#

Meaning: A ValueError occurs when an operation or function receives an argument of the right type but inappropriate value. For instance, converting a string that doesn’t represent a valid number into an integer will raise a ValueError.

How to Address:

  • Validate Inputs: Ensure that the input values are in the correct format before processing them. For example, if expecting a number, confirm that the input string can be converted to an integer or float.

  • Use Exception Handling: Employ try and except blocks to manage unexpected values gracefully and provide useful error messages or fallback behavior.

ZeroDivisionError#

Meaning: This error is raised when a division or modulo operation is attempted with a divisor of zero. In mathematics, division by zero is undefined, and Python prevents this operation to avoid producing an invalid result.

How to Address:

  • Ensure that the divisor is not zero before performing a division or modulo operation. This can be done by adding a conditional check to handle cases where the divisor might be zero, and either skipping the operation or providing an alternative result.

  • Exception Handling: Wrap the division access in a try-except block to catch the ZeroDivisionError and handle it appropriately. Example:

try:
    value = 10/0
except KeyError:
    value = None
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