Why does this statement return a syntax error

Ситуация: программист взял в работу математический проект — ему нужно написать код, который будет считать функции и выводить результаты. В задании написано:

«Пусть у нас есть функция f(x,y) = xy, которая перемножает два аргумента и возвращает полученное значение».

Программист садится и пишет код:

a = 10
b = 15
result = 0
def fun(x,y): 
    return x y
result = fun(a,b)
print(result)

Но при выполнении такого кода компьютер выдаёт ошибку:

File "main.py", line 13
result = x y
^
❌ SyntaxError: invalid syntax

Почему так происходит: в каждом языке программирования есть свой синтаксис — правила написания и оформления команд. В Python тоже есть свой синтаксис, по которому для умножения нельзя просто поставить рядом две переменных, как в математике. Интерпретатор находит первую переменную и думает, что ему сейчас объяснят, что с ней делать. Но вместо этого он сразу находит вторую переменную. Интерпретатор не знает, как именно нужно их обработать, потому что у него нет правила «Если две переменные стоят рядом, их нужно перемножить». Поэтому интерпретатор останавливается и говорит, что у него лапки. 

Что делать с ошибкой SyntaxError: invalid syntax

В нашем случае достаточно поставить звёздочку (знак умножения в Python) между переменными — это оператор умножения, который Python знает:

a = 10
b = 15
result = 0
def fun(x,y): 
    return x * y
result = fun(a,b)
print(result)

В общем случае найти источник ошибки SyntaxError: invalid syntax можно так:

Практика

Попробуйте найти ошибки в этих фрагментах кода:

x = 10 y = 15
def fun(x,y): 
    return x * y

try:  
    a = 100
    b = "PythonRu"
    assert a = b
except AssertionError:  
    print("Исключение AssertionError.")
else:  
    print("Успех, нет ошибок!")

Software errors are prevalent. It’s easy to make them, and it’s hard to find them. In this chapter, we’ll explore topics related to the finding and removal of bugs within our C++ programs, including learning how to use the integrated debugger that is part of our IDE.

Although debugging tools and techniques aren’t part of the C++ standard, learning to find and remove bugs in the programs you write is an extremely important part of being a successful programmer. Therefore, we’ll spend a bit of time covering such topics, so that as the programs you write become more complex, your ability to diagnose and remedy issues advances at a similar pace.

If you have experience from debugging programs in another compiled programming language, much of this will be familiar to you.

Syntax and semantic errors

Programming can be challenging, and C++ is somewhat of a quirky language. Put those two together, and there are a lot of ways to make mistakes. Errors generally fall into one of two categories: syntax errors, and semantic errors (logic errors).

A syntax error occurs when you write a statement that is not valid according to the grammar of the C++ language. This includes errors such as missing semicolons, using undeclared variables, mismatched parentheses or braces, etc… For example, the following program contains quite a few syntax errors:

#include <iostream>

int main()
{
    std::cout < "Hi there"; << x << 'n'; // invalid operator (<), extraneous semicolon, undeclared variable (x)
    return 0 // missing semicolon at end of statement
}

Fortunately, the compiler will generally catch syntax errors and generate warnings or errors, so you easily identify and fix the problem. Then it’s just a matter of compiling again until you get rid of all the errors.

Once your program is compiling correctly, getting it to actually produce the result(s) you want can be tricky. A semantic error occurs when a statement is syntactically valid, but does not do what the programmer intended.

Sometimes these will cause your program to crash, such as in the case of division by zero:

#include <iostream>

int main()
{
    int a { 10 };
    int b { 0 };
    std::cout << a << " / " << b << " = " << a / b << 'n'; // division by 0 is undefined
    return 0;
}

More often these will just produce the wrong value or behavior:

#include <iostream>

int main()
{
    int x;
    std::cout << x << 'n'; // Use of uninitialized variable leads to undefined result

    return 0;
}

or

#include <iostream>

int add(int x, int y)
{
    return x - y; // function is supposed to add, but it doesn't
}

int main()
{
    std::cout << add(5, 3) << 'n'; // should produce 8, but produces 2

    return 0;
}

or

#include <iostream>

int main()
{
    return 0; // function returns here

    std::cout << "Hello, world!n"; // so this never executes
}

Modern compilers have been getting better at detecting certain types of common semantic errors (e.g. use of an uninitialized variable). However, in most cases, the compiler will not be able to catch most of these types of problems, because the compiler is designed to enforce grammar, not intent.

In the above example, the errors are fairly easy to spot. But in most non-trivial programs, semantic errors are not easy to find by eyeballing the code. This is where debugging techniques can come in handy.

“SyntaxError: Illegal return statement” in JavaScript is a syntax-related error. It describes that you are typing the command wrong or using the statement in the wrong place. Please read the following instructions to resolve it.

The most common reason for “SyntaxError: Illegal return statement” in Javascript is that you used the return statement incorrectly. As the error code describes, it’s a syntax-related error, indicating it’s a return statement. See the example below to better understand.

Here I have a very simple example: if the condition in the learn() function is true, then a corresponding message is sent. But if I leave the return statement out of the function, it throws an error like this:

var slogan = "LearnShareIT"

function learn() {
    if (slogan == "LearnShareIT") {        
        return "I am a developer!!";        
    }  
}

//returns in the wrong place
return "I am a student!!";

console.log(learn());

Output

main.js:173 Uncaught SyntaxError: Illegal return statement (at main.js:173:1)

Or if I am missing a curly brace, it still gives me the same error:

var slogan = "LearnShareIT"

function learn() //missing here{        
    return slogan;        
}

console.log(learn());

Output

Uncaught SyntaxError: Illegal return statement (at main.js:171:5)

Since a return statement follows the curly brace, it will still report an error.

To return an appropriate value to the function without causing an error, refer to the way below.

How to fix this error?

Use the correct command structure

Also the above example, I proceed to reset the position of the return command, and immediately it shows the result I want as follows:

var slogan = "LearnShareIT"

function learn() {
    if (slogan == "LearnShareIT") {        
        return "I am a developer!!";        
    }
  
//put the return command in the right place
    return "I am a student!!";
}

console.log(learn());

Output

I am a developer!!

For the missing function brackets, we need to add the correct syntax, and the code will run smoothly:

var slogan = "LearnShareIT"

function learn() {        
    return slogan;        
}

console.log(learn());

Output

LearnShareIT

Shorten the number of return statements

Too many return statements in a function will cause trouble for your code. Return is a command directly related to the computer’s memory. If you abuse it, you will get an error.

Example

var fruit = 'apple';

function learn() {
    if (fruit == 'apple') {
        return 'apple';
    } 
    else {
        return 'banana';
    }

//returns in the wrong place
    return 'orange';

//returns in the wrong place
}return 'melon';

Output

main.js:186 Uncaught SyntaxError: Illegal return statement (at main.js:186:2)

Using too many return statements will mess up the code and you might end up putting it in the wrong place, so shorten it like this:

var fruit = 'apple';

function choose() {
    if (fruit == 'apple') {
        return 'apple';
    } 
    else {
        return 'banana';
    }
}

console.log(choose());

Output

apple

Summary

For the error “SyntaxError: Illegal return statement” in JavaScript, you need to understand what you want your function to return so you can use return correctly. Using return indiscriminately poses a huge risk to your code, so be aware. Above are solutions to help you solve this error, good luck with your studies.

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• TypeError: indexOf is not a function in JavaScript
• SyntaxError: Unexpected token in JavaScript