Understanding Assertion Errors In MicroPython Socket Calls

Introduction

Hey guys! Today, we're diving deep into a rather intriguing issue encountered in MicroPython: an assertion error triggered by a bad call to socket.socket(). Specifically, this problem arises when we try to instantiate a socket with an invalid argument. Now, you might be wondering, “What exactly constitutes a bad call?” Well, in this case, it involves passing a non-integer value (specifically, a boolean True) as the first argument to the socket.socket() function. This might seem like a small oversight, but it leads to a rather significant hiccup within MicroPython's internal workings, causing an assertion failure. Let’s break down the situation, understand the error, explore why it happens, and discuss what the expected behavior should be.

When working with MicroPython, especially in embedded systems and IoT projects, the socket module is your gateway to network communication. It allows your device to connect to networks, send data, and receive responses, making it a cornerstone of many applications. However, like any powerful tool, it requires precise usage. The socket.socket() function, the heart of this module, expects certain parameters to be of specific types. For instance, the first argument, which specifies the address family, is expected to be an integer. When we deviate from this expectation, we risk encountering errors like the one we’re discussing today. So, let's roll up our sleeves and get into the nitty-gritty details of this assertion error and how to avoid it.

The issue, at its core, stems from a type mismatch. MicroPython, being a lean and efficient implementation of Python 3 designed for microcontrollers, has certain expectations about the data types it receives. When these expectations aren't met, it can lead to unexpected behavior, including assertion failures. Understanding these expectations and how to adhere to them is crucial for writing robust and reliable MicroPython code. So, stick with me as we dissect this problem, explore the code snippet that triggers it, and delve into the implications for your MicroPython projects. We'll also touch on how this issue manifests in a specific environment (unix port, coverage variant, x86_64 linux) and the particular version of MicroPython where it was observed (MicroPython v1.26.0-preview.521.g658a2e3dbd on 2025-08-02; linux [GCC 12.2.0] version).

Reproducing the Error

To get our hands dirty and really understand the problem, let's look at the code snippet that triggers this assertion error. It’s surprisingly simple, which makes the underlying issue all the more interesting. Here’s the code:

>>> import socket
>>> socket.socket(True)
micropython: modsocket.c:481: socket_make_new: Assertion `mp_obj_is_small_int(args[0])' failed.

As you can see, we're importing the socket module and then attempting to create a socket using socket.socket(True). The critical part here is the True argument. In the context of the socket.socket() function, the first argument is expected to specify the address family, which should be an integer value like socket.AF_INET (for IPv4) or socket.AF_INET6 (for IPv6). By passing True, which is a boolean, we're violating this expectation. This leads to the assertion failure deep within MicroPython's C code, specifically in the socket_make_new function within modsocket.c.

The error message, micropython: modsocket.c:481: socket_make_new: Assertion gravemp_obj_is_small_int(args[0])' failed., is quite telling. It indicates that the assertion mp_obj_is_small_int(args[0]) failed. Let's break this down:

• modsocket.c:481: This tells us the exact file and line number in the MicroPython source code where the error occurred. This is invaluable for developers debugging the MicroPython implementation itself.
• socket_make_new: This is the function within the socket module that's responsible for creating new socket objects. It's the entry point where the arguments passed to socket.socket() are processed.
• Assertion gravemp_obj_is_small_int(args[0])' failed.: This is the heart of the matter. Assertions are checks that are expected to be true at a certain point in the code. If an assertion fails, it means something has gone wrong. In this case, mp_obj_is_small_int(args[0]) is a function (or macro) that checks if the first argument (args[0]) is a small integer. The fact that this assertion failed indicates that args[0] (which is our True value) is not a small integer.

This example highlights the importance of understanding the expected data types for function arguments. MicroPython, while being a flexible language, still relies on certain type constraints for its underlying C implementation to function correctly. By providing the wrong type, we're essentially stepping outside the boundaries of what the code is designed to handle, leading to this assertion error. So, the key takeaway here is to always ensure that you're passing the correct data types to functions, especially when dealing with low-level modules like socket.

Expected vs. Observed Behavior

Let's talk about what should happen versus what actually happens when we make this bad call to socket.socket(). Ideally, in a well-behaved Python environment, passing an invalid argument type to a function should raise a Python exception. This is a standard mechanism for error handling in Python, allowing the program to gracefully catch and handle issues without crashing. Specifically, in this case, we'd expect a TypeError to be raised, indicating that the argument type is incorrect.

A TypeError is a built-in exception in Python that's raised when an operation or function is applied to an object of inappropriate type. For example, trying to add a string to an integer will raise a TypeError. In the context of our socket.socket() call, passing a boolean where an integer is expected clearly falls under this category. A TypeError would provide a clear and informative message to the user, such as