Troubleshooting Guide: Resolving SystemError - New Style GetArgs Format but Argument is not a Tuple

This troubleshooting guide will help you resolve the SystemError: new style getargs format but argument is not a tuple error generated in Python. We will provide a step-by-step solution to help you understand the cause of the error and how to fix it.

Table of Contents

Understanding the Error

Before diving into the solution, it is essential to understand the cause of the error. The SystemError: new style getargs format but argument is not a tuple error occurs when you are trying to parse arguments in a Python function using the PyArg_ParseTuple or PyArg_ParseTupleAndKeywords function, but the argument passed is not in the correct format (i.e., not a tuple).

The error is usually encountered while working with C extensions for Python or while calling Python functions from C or C++ code. This error can be frustrating, as it often occurs due to a minor mistake in the argument format.

Step-by-Step Solution

Follow these steps to resolve the error:

Identify the problematic function: Locate the function where the error is being raised. This is usually a function called from C or C++ code that is trying to parse Python arguments using PyArg_ParseTuple or PyArg_ParseTupleAndKeywords.

Check the argument format: Ensure the argument passed to the function is in the correct format, i.e., a tuple. If the argument is a single value, make sure to pass it as a one-element tuple, like this: (arg,).

Verify the format string: Check the format string used in PyArg_ParseTuple or PyArg_ParseTupleAndKeywords and ensure it matches the expected format of the arguments. The Python/C API documentation provides a detailed explanation of the format codes.

Update the function call: If the argument format is incorrect, update the function call to pass the arguments as a tuple. For example, if the function call looks like this:

PyObject_CallFunction(func, "O", arg);

Update it to:

PyObject_CallFunction(func, "(O)", arg);

Test your changes: Compile and run your code to ensure the error has been resolved.


1. What is the purpose of PyArg_ParseTuple and PyArg_ParseTupleAndKeywords?

These functions are part of the Python/C API and are used to parse arguments passed from Python to C or C++ functions. They convert Python objects into C values based on the specified format strings.

2. Can this error occur when using ctypes or cffi?

Yes, this error can occur when using ctypes or cffi if the argument format is not correct when calling a C function from Python.

3. Are there alternatives to using PyArg_ParseTuple and PyArg_ParseTupleAndKeywords?

Yes, there are alternatives such as using the PyArg_UnpackTuple function, which can parse arguments more flexibly. However, these functions still require the correct argument format (i.e., a tuple).

4. How can I avoid this error in the future?

Always ensure that the arguments are passed as tuples when calling functions that use PyArg_ParseTuple or PyArg_ParseTupleAndKeywords. Double-check the format strings to ensure they match the expected format of the arguments.

5. Can this error occur in Python 2.x?

Yes, this error can occur in both Python 2.x and Python 3.x, as the underlying cause is the same - passing an incorrectly formatted argument.

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