Invalid Syntax in Python: Common Reasons for SyntaxError

Invalid Syntax in Python

When you run your Python code, the interpreter will first parse it to convert it into Python byte code, which it will then execute. The interpreter will find any invalid syntax in Python during this first stage of program execution, also known as the parsing stage. If the interpreter can’t parse your Python code successfully, then this means that you used invalid syntax somewhere in your code. The interpreter will attempt to show you where that error occurred.

When you’re learning Python for the first time, it can be frustrating to get a SyntaxError. Python will attempt to help you determine where the invalid syntax is in your code, but the traceback it provides can be a little confusing. Sometimes, the code it points to is perfectly fine.

You can’t handle invalid syntax in Python like other exceptions. Even if you tried to wrap a try and except block around code with invalid syntax, you’d still see the interpreter raise a SyntaxError.

SyntaxError Exception and Traceback

When the interpreter encounters invalid syntax in Python code, it will raise a SyntaxError exception and provide a traceback with some helpful information to help you debug the error. Here’s some code that contains invalid syntax in Python:

 1# theofficefacts.py
 2ages = {
 3    'pam': 24,
 4    'jim': 24
 5    'michael': 43
 6}
 7print(f'Michael is {ages["michael"]} years old.')

You can see the invalid syntax in the dictionary literal on line 4. The second entry, 'jim', is missing a comma. If you tried to run this code as-is, then you’d get the following traceback:

$ python theofficefacts.py
File "theofficefacts.py", line 5
    'michael': 43
            ^
SyntaxError: invalid syntax

Note that the traceback message locates the error in line 5, not line 4. The Python interpreter is attempting to point out where the invalid syntax is. However, it can only really point to where it first noticed a problem. When you get a SyntaxError traceback and the code that the traceback is pointing to looks fine, then you’ll want to start moving backward through the code until you can determine what’s wrong.

In the example above, there isn’t a problem with leaving out a comma, depending on what comes after it. For example, there’s no problem with a missing comma after 'michael' in line 5. But once the interpreter encounters something that doesn’t make sense, it can only point you to the first thing it found that it couldn’t understand.

There are a few elements of a SyntaxError traceback that can help you determine where the invalid syntax is in your code:

• The file name where the invalid syntax was encountered
• The line number and reproduced line of code where the issue was encountered
• A caret (^) on the line below the reproduced code, which shows you the point in the code that has a problem
• The error message that comes after the exception type SyntaxError, which can provide information to help you determine the problem

In the example above, the file name given was theofficefacts.py, the line number was 5, and the caret pointed to the closing quote of the dictionary key michael. The SyntaxError traceback might not point to the real problem, but it will point to the first place where the interpreter couldn’t make sense of the syntax.

There are two other exceptions that you might see Python raise. These are equivalent to SyntaxError but have different names:

1. IndentationError
2. TabError

These exceptions both inherit from the SyntaxError class, but they’re special cases where indentation is concerned. An IndentationError is raised when the indentation levels of your code don’t match up. A TabError is raised when your code uses both tabs and spaces in the same file. You’ll take a closer look at these exceptions in a later section.

Common Syntax Problems

When you encounter a SyntaxError for the first time, it’s helpful to know why there was a problem and what you might do to fix the invalid syntax in your Python code. In the sections below, you’ll see some of the more common reasons that a SyntaxError might be raised and how you can fix them.

>>> len('hello') = 5
  File "<stdin>", line 1
SyntaxError: can't assign to function call

>>> 'foo' = 1
  File "<stdin>", line 1
SyntaxError: can't assign to literal

>>> 1 = 'foo'
  File "<stdin>", line