Python - Syntax Errors

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Generally, three types of errors appear in a computer program: Syntax errors, logical errors and runtime errors. Syntax errors are the most common type of errors one faces while writing a program, whether you are new to programming or an experienced programmer. Syntax errors are basically related to the rules of grammar of a certain language.

Syntax errors occur whenever the rules laid down by the language are not followed. In Python, there are well defined rules for giving name to an identifier, that is, a variable, a function, a class, a module or any Python object. Similarly, Python keywords should be used as per the syntax defined. Whenever these rules are not followed, Python interpreter displays a syntax error message.

A simple example of declaring a variable in Python interactive shell is given below.

>>> name="Python
   File "<stdin>", line 1
      name="Python
           ^
SyntaxError: unterminated string literal (detected at line 1)

Python interpreter displays syntax error along with a certain explanatory message. In the above example, because the quotation symbol is not closed, the Syntax error occurs.

Similarly, Python requires each function name should be followed by parantheses inside which the function arguments should be given.

In the following example, we get a syntax error −

>>> print "Hello"
   File "<stdin>", line 1
      print "Hello"
      ^^^^^^^^^^^^^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(...)?

The reason can be understood from the error message, that the print() function is missing parentheses.

There are many popular IDEs for Python programming. Most of them use colorized syntax highlighting, which makes it easy to visually identify the error.

One such IDE is VS Code. While entering an instruction, the syntax errors are suitably highlighted.

The error is highlighted. If you put the cursor there, VS Code tells more about the error. If you still go ahead and execute the code, error messages appear in the command terminal.

Syntax errors are easy to identify and rectify. The IDE such as VS Code makes it easy. However, sometimes, your code doesn't show any syntax errors, but still the output of the program is not what you anticipate. Such errors are logical errors. They are hard to detect, as the error lies in the logic used in the code. You learn by experience how to correct logical errors. VS Code and other IDEs have features such as watches and breakpoints to trap these errors.

Third type of error is a runtime error also called exception. There is no syntax error nor there is any logical error in your program. Most of the times, the program gives desired output, but in some specific situations you get abnormal behaviour of the program, such as the program abnormally terminates or gives some absurd result.

The factors causing exceptions are generally external to the program. For example incorrect input, type conversion or malfunction IO device etc.

What is Exception?

An exception is an event, which occurs during the execution of a program that disrupts the normal flow of the program's instructions. In general, when a Python script encounters a situation that it cannot cope with, it raises an exception. An exception is a Python object that represents an error.

When a Python script raises an exception, it must either handle the exception immediately otherwise it terminates and quits.

Python's standard library defines standard exception classes. As with other Python classes, Exceptions are also subclasses of Object class. Following is the object hierarchy of Python's Exceptions.

object
   BaseException
      Exception
         ArithmeticError
            FloatingPointError
            OverflowError
            ZeroDivisionError
         AssertionError
         AttributeError
         BufferError
         EOFError
         ImportError
            ModuleNotFoundError
         LookupError
            IndexError
            KeyError
         MemoryError
         NameError
         OSError
         ReferenceError
         RuntimeError
         StopAsyncIteration
         StopIteration
         SyntaxError