Thursday, April 18, 2019

Python's dynamic nature: sticking an attribute onto an object

- By Vasudev Ram - Online Python training / SQL training / Linux training

Hi, readers,

[This is a beginner-level Python post.]

Python, being a dynamic language, has some interesting features that some static languages may not have (and vice versa too, of course).

One such feature, which I noticed a while ago, is that you can add an attribute to a Python object even after it has been created. (Conditions apply.)

I had used this feature some time ago to work around some implementation issue in a rudimentary RESTful server that I created as a small teaching project. It was based on the BaseHTTPServer module.

Here is a (different) simple example program,, that demonstrates this Python feature.
My informal term for this feature is "sticking an attribute onto an object" after the object is created.

Since the program is simple, and there are enough comments in the code, I will not explain it in detail.

# A program to show:
# 1) that you can "stick" attributes onto a Python object after it is created, and
# 2) one use of this technique, to count the number# of calls to a function.

# Copyright 2019 Vasudev Ram
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from __future__ import print_function

# Define a function.
def foo(arg):
    # Print something to show that the function has been called.
    print("in foo: arg = {}".format(arg))
    # Increment the "stuck-on" int attribute inside the function.
    foo.call_count += 1

# A function is also an object in Python.
# So we can add attributes to it, including after it is defined.
# I call this "sticking" an attribute onto the function object.
# The statement below defines the attribute with an initial value, 
# which is changeable later, as we will see.
foo.call_count = 0

# Print its initial value before any calls to the function.
print("foo.call_count = {}".format(foo.call_count))

# Call the function a few times.
for i in range(5):

# Print the attribute's value after those calls.
print("foo.call_count = {}".format(foo.call_count))

# Call the function a few more times.
for i in range(3):

# Print the attribute's value after those additional calls.
print("foo.call_count = {}".format(foo.call_count))

And here is the output of the program:
$ python
foo.call_count = 0
in foo: arg = 0
in foo: arg = 1
in foo: arg = 2
in foo: arg = 3
in foo: arg = 4
foo.call_count = 5
in foo: arg = 0
in foo: arg = 1
in foo: arg = 2
foo.call_count = 8

There may be other ways to get the call count of a function, including using a profiler, and maybe by using a closure or decorator or other way. But this way is really simple. And as you can see from the code, it is also possible to use it to find the number of calls to the function, between any two points in the program code. For that, we just have to store the call count in a variable at the first point, and subtract that value from the call count at the second point. In the above program, that would be 8 - 5 = 3, which matches the 3 that is the number of calls to function foo made by the 2nd for loop.


- Vasudev Ram - Online Python training and consulting

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