Error messages

Each of you has certainly encountered error messages. For example, issuing the command in the console:

print(float("two"))

It will produce the message:

Traceback (most recent call last):

  File "<ipython-input-1-ed629b1feb04>", line 1, in <module>
    print(float("two"))

ValueError: could not convert string to float: 'two'

In Python, errors are called exceptions. Their appearance means that a non-standard situation has occurred. Its appearance may result from an error in the program: then the message shows the exact place where the problem appeared and the entire sequence of function calls that led to it. Please take a look at the following program (when rewriting it, please skip the line numbers that are given for information only):

1 def replace_number(value):
2 return float(value)
3
4 print(replace_number('2'))
5   print(replace_number('two'))

Its launch will result in the following message:

Traceback (most recent call last):

  File "temp.py", line 5, in <module>
    print (replace_number("two"))

  File "temp.py", line 2, in replace_number
    return float(value)

ValueError: could not convert string to float: 'two'

Python printed the so-called traceback, which is the function call stack. It should be read from the bottom: an error has occurred ValueError, i.e. a value error. We read in the error message that Python is not able to convert the text string 'two' to a real number. Above, we find information that this error occurred in the second line of the file temp.py in the function replace_number (this line is rewritten below). In turn, this function was called on the fifth line in the main code of the module (without any function). Note that calling the function replace_number on line four did not produce an error.

Exception types

In Python, exceptions can be of different types that correspond to different types of errors or abnormal situations. The following is a list of the most common types of exceptions:

AttributeError: Raised when attribute reference or assignment fails (when the object does not support attribute references or assignments at all TypeError).

IOError: Dispatched when an error occurs in the I/O of a print statement, built-in function, open() or file object methods. For example, these errors can be caused by “file not found” or “out of disk space” problems.

ImportError: Raised when an import statement fails because the module is not found, or when the from ... import statement cannot find the specified name in the module.

IndexError: Called when an index out of scope is used to access an element of a sequence (however, mind that slices are automatically adjusted to the size of the sequence; if the index is not an integer TypeError is raised).

KeyError: Raised when the key used to access the mapping element (dictionary) is not present.

KeyboardInterrupt: Called when the interrupt key is used (usually Ctrl-C or Ctrl-Delete). Checking for this is done regularly when the program is called. Using the abort key (key combination) while the function is waiting for input in the input() function also raises this exception.

MemoryError: Raised when the memory is exhausted. The value assigned to the exception is a string indicating what (internal) operation has exhausted memory.

NameError: Raised when local or global name is not found. The value associated with the exception is an error message that contains a name that was not found.

SyntaxError: Raised when the parser encounters a syntax error.

TypeError: Raised when an inline operation is performed on an object of the wrong type. The assigned value of the exception is a string that describes the details of the type mismatch.

UnboundLocalError: Raised when reference is made to a local variable of a function or method and no value has been assigned to that variable.

ValueError: Raised when some operation was requested with an argument of the right type, but with an incorrect value.

ZeroDivisionError: Raised when the second operand of the division operation or the division remainder (modulo) is zero. The value assigned to the exception is a string that describes the types of operands and operations.

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