3.5. Syntax errorsΒΆ

Python can only execute a program if the program is syntactically correct; otherwise, the process fails and returns an error message. Syntax refers to the structure of a program and the rules about that structure. For example, in English, a sentence must begin with a capital letter and end with a period. this sentence contains a syntax error. So does this one

In Python, rules of syntax include requirements like these: strings must be enclosed in quotes; statements must generally be written one per line; the print statement must enclose the value to be displayed in parenthesis; expressions must be correctly formed. The following lines contain syntax errors:

print(Hello, world!)
print "Hello, world!"
print(5 + )

For most readers of English, a few syntax errors are not a significant problem, which is why we can read the poetry of e. e. cummings without problems. Python is not so forgiving. When you run a Python program, the interpreter checks it for syntax errors before beginning to execute the first statement. If there is a single syntax error anywhere in your program, Python will display an error message and quit without executing any of the program.

To see a syntax error in action, look at the following program. Can you spot the error? After locating the error, run the program to see the error message.

Notice the following:

  1. The error message clearly indicates that the problem is a SyntaxError. This lets you know the problem is not one of the other two types of errors we’ll discuss shortly.

  2. The error is on line 2 of the program. However, even though there is nothing wrong with line 1, the print statement does not execute β€” none of the program successfully executes because of the presence of just one syntax error.

  3. The error gives the line number where Python believes the error exists. In this case, the error message pinpoints the location correctly. But in other cases, the line number can be inaccurate or entirely missing.

    To see an example of the latter, try removing just the right parenthesis ) from line 2 and running the program again. Notice how the error message gives no line number at all. With syntax errors, you need to be prepared to hunt around a bit in order to locate the trouble.

One aspect of syntax you have to watch out for in Python involves indentation. Python requires you to begin all statements at the beginning of the line, unless you are using a flow control statement like a for or an if statement (we’ll discuss these soon… stay tuned!). To see an example of this kind of problem, modify the program above by inserting a couple of spaces at the beginning of one of the lines.

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