Python sys.settrace() Method

Python sys.settrace() Method

  1. Syntax of Python sys.settrace() Method
  2. Example Codes: Use the sys.settrace() Method in Python
  3. Example Codes: Call a Loop Function Recursively Using the sys.settrace() Method

Python sys.settrace() method is an efficient way of registering the traceback to the platform’s Python interpreter. A traceback call is the information returned when an event occurs in the code.

Syntax of Python sys.settrace() Method

sys.settrace(frame, event, arg)

Parameters

frame It represents the current stack frame.
event Keywords of string data type, such as call, line, return, exception, or opcode. The events have the following meaning.
call: A function or a block of code is called.
line: A line of code or a loop is about to be executed.
return: A function or a block of code is returned.
exception: An exception has occurred.
opcode: A new opcode is to be executed by the interpreter.
arg The type of event that has occurred. The events have the following arguments.
return: The value to be returned or none.
exception: The argument is a tuple consisting of exception, value, and traceback.

Return

The return type of this method is a reference to the local trace function, which returns a reference to itself after execution.

Example Codes: Use the sys.settrace() Method in Python

from sys import settrace

def tracer(frame, event, arg = None):

    code = frame.f_code
    func_name = code.co_name
    line_no = frame.f_lineno

    print(f"A {event} has encountered in\
    {func_name}() at the following line number {line_no}. ")

    return tracer

def function():
    return "Python"

def system():
    return function()

settrace(tracer)
system()

Output:

A call has encountered in       system() at the following line number 19.
A line has encountered in       system() at the following line number 20.
A call has encountered in       function() at the following line number 16.
A line has encountered in       function() at the following line number 17.
A return has encountered in     function() at the following line number 17.
A return has encountered in     system() at the following line number 20.

We used global trace functions in the above code to implement the local trace function, which returns itself.

Example Codes: Call a Loop Function Recursively Using the sys.settrace() Method

import sys
from sys import settrace

def trace(frame , event, arg):

    code = frame.f_locals["a"]

    if code % 2 == 0:
        frame.f_locals["a"] = code

def f(a):

    print (a)

if __name__ == "__main__":

    sys.settrace(trace)
    for x in range(0,10):
        f(x)

Output:

0
1
2
3
4
5
6
7
8
9

In the above code, we modified the arguments sent to a function. The function uses a for loop and counts the digit in a specified range.

Musfirah Waseem avatar Musfirah Waseem avatar

Musfirah is a student of computer science from the best university in Pakistan. She has a knack for programming and everything related. She is a tech geek who loves to help people as much as possible.

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