How to Use the sys.settrace to Debug in Python?


sys.settrace is a built-in function in Python’s sys module. It allows you to set a trace function that is called by the Python interpreter for every line of code that is executed. The trace function can be used to monitor the execution of Python code, inspect variables and control flow, and even modify the code being executed.

The syntax of sys.settrace is as follows:

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sys.settrace(tracefunc)
sys.settrace(tracefunc)

Here, tracefunc is a callable that is called by the Python interpreter for each line of code that is executed. The tracefunc should accept three arguments: frame, event, and args. frame is the frame object representing the current execution frame, event is a string indicating the type of event that triggered the trace function (e.g., “line”, “call”, “return”), and args is a tuple containing additional event-specific arguments.

Here is an example of a simple trace function that prints the file name, line number, and code for each executed line:

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import sys
 
def tracefunc(frame, event, args):
    if event == "line":
        filename = frame.f_code.co_filename
        lineno = frame.f_lineno
        line = linecache.getline(filename, lineno)
        print(f"{filename}:{lineno}: {line.strip()}")
 
sys.settrace(tracefunc)
 
# Your Python code goes here...
import sys

def tracefunc(frame, event, args):
    if event == "line":
        filename = frame.f_code.co_filename
        lineno = frame.f_lineno
        line = linecache.getline(filename, lineno)
        print(f"{filename}:{lineno}: {line.strip()}")

sys.settrace(tracefunc)

# Your Python code goes here...

Note that the tracefunc should return itself if you want to continue tracing, or None if you want to stop tracing.

It is important to use sys.settrace carefully and avoid creating infinite loops or causing performance issues. Additionally, some Python implementations may limit the use of sys.settrace for security reasons.

Summary of Using sys.settrace in Python

Python’s sys.settrace is a handy function to debug Python code. Do use it with precaution as it will slow down the Python code.

–EOF (The Ultimate Computing & Technology Blog) —

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