Tag Archives: Overflow

Some weird things from Python 3 that you may not know

Last Update: 2021-09-29 18:12 IST

You can find those bizarre things and more in my book Python 3 Combat Guide.

I’m not talking about the wonderful things, like how big can the Integers be, but about the bizarre things that may ruin your day.

What sums 0.1 + 0.1 + 0.1 in Python?

0.3?

Wrong answer.

A bit of humor

Well, to be honest the computer was wrong. They way programming languages handle the Floats tend to be less than ideal.

Floats

Maybe you know JavaScript and its famous NaN (Not a number).

You are probably sure that Python is much more exact than that…

…well, until you do a big operation with Floats, like:

10.0**304 * 10.0**50 

and

It returns infinite

I see your infinite and I add one :)

However If we try to define a number too big directly it will return OverflowError:

Please note Integers are handled in a much more robust cooler way:

Negative floats

Ok. What happens if we define a number with a negative power, like 10 ** -300 ?

And if we go somewhere a bit more far? Like 10 ** -329

It returns 0.0

Ups!

I mention in my books why is better to work with Integers, and in fact most of the eCommerces, banks and APIs work with Integers. For example, if the amount in USD 10.00 they send multiplied by 100, so they will send 1000. All the actor know that they have to divide by 2.

Breaking the language innocently

I mentioned always that I use the MT Notation, the prefix notation I invented, inspired by the Hungarian Notation and by an amazing C++ programmer I worked with in Volkswagen and in la caixa (now caixabank), that passed away many years ago.

Well, that system of prefixes will name a variable with a prefix for its type.

It’s very useful and also prevents the next weird thing from Python.

Imagine a Junior wants to print a String and they put in a variable. And unfortunately they call this variable print. Well…

print = "Hello World!"
print("That will hurt")

Observe the output of this and try not to scream:

Variables and Functions named equally

Well, most of languages are able to differentiate a function, with its parenthesis, from a variable.

The way Python does it hurts my coder heart:

Another good reason to use MT Notation for the variables, and for taking seriously doing Unit Testing and giving a chance to using getters and setters and class Constructor for implementing limits and sanitation.

Nested Loops

This will work in Python, it doesn’t work in other languages (but please never do it).

for i in range(3):
    print("First Loop", i)
    for i in range(4):
        print("Second Loop", i)

The code will not crash by overwriting i used in the first loop, but the new i will mask the first variable.

And please, name variables properly.

Import… once?

Imports are imported only once. Even if different files imported do import the same file.

So don’t have code in the middle of them, outside functions/classes, unless you’re really know what you’re doing.

Define functions first, and execute code after if __name__ == “__main__”:

Take a look at this code:

def first_function():
    print("Inside first function")
    second_function()

first_function()

def second_function():
    print("Inside second function")

Well, this will crash as Python executes the code from top to bottom, and when it gets to first_function() it will attempt to call second_function() which has not been read by Python yet. This example will throw an error.

You’ll get an error like:

Inside first function
Traceback (most recent call last):
  File "/home/carles/Desktop/code/carles/python_combat_guide/src/structure_dont_do_this.py", line 14, in <module>
    first_function()
  File "/home/carles/Desktop/code/carles/python_combat_guide/src/structure_dont_do_this.py", line 12, in first_function
    second_function()
NameError: name 'second_function' is not defined

Process finished with exit code 1

Add your code at the bottom always, under:

if __name__ == "__main__":
    main()

The code inside this if will only be executed if you directly call this code as main file, but will not be executed if you import this file from another one.

You don’t have this problem with classes in Python, as they are defined first, completely read, and then you instantiate or use them. To avoid messing and creating bugs, have the imports always on the top of your file.