Python vs C++: Selecting the Right Tool for the Job

Whitespace

The first thing most developers notice when comparing Python vs C++ is the “whitespace issue.” Python uses leading whitespace to mark scope. This means that the body of an if block or other similar structure is indicated by the level of indentation. C++ uses curly braces ({}) to indicate the same idea.

While the Python lexer will accept any whitespace as long as you’re consistent, PEP8 (the official style guide for Python) specifies 4 spaces for each level of indentation. Most editors can be configured to do this automatically.

There has been an enormous amount of writing, shouting, and ranting about Python’s whitespace rules already, so let’s just jump past that issue and on to other matters.

Instead of relying on a lexical marker like ; to end each statement, Python uses the end of the line. If you need to extend a statement over a single line, then you can use the backslash (\) to indicate that. (Note that if you’re inside a set of parentheses, then the continuation character is not needed.)

There are people who are unhappy on both sides of the whitespace issue. Some Python developers love that you don’t have to type out braces and semicolons. Some C++ developers hate the reliance on formatting. Learning to be comfortable with both is your best bet.

Now that you’ve looked at the whitespace issue, let’s move on to one that’s a bit less contentious: Boolean expressions.

Boolean Expressions

The way you’ll use Boolean expressions changes slightly in Python vs C++. In C++, you can use numeric values to indicate true or false, in addition to the built-in values. Anything that evaluates to 0 is considered false, while every other numeric value is true.

Python has a similar concept but extends it to include other cases. The basics are quite similar. The Python documentation states that the following items evaluate to False:

• Constants defined as false:
  • None
  • False
• Zeros of any numeric type:
  • 0
  • 0.0
  • 0j
  • Decimal(0)
  • Fraction(0, 1)
• Empty sequences and collections:
  • ''
  • ()
  • []
  • {}
  • set()
  • range(0)

All other items are True. This means that an empty list [] is False, while a list containing only zero [0] is still True.

Most objects will evaluate to True, unless the object has __bool__() which returns False or __len__() which returns 0. This allows you to extend your custom classes to act as Boolean expressions.

Python has a few slight changes from C++ in the Boolean operators as well. For starters, if and while statements do not require the surrounding parentheses as they do in C++. Parentheses can aid in readability, however, so use your best judgment.

Most C++ Boolean operators have similar operators in Python:

Most of the operators are similar to C++, but if you want to brush up you can read Operators and Expressions in Python.

Variables and Pointers

When you first start using Python after writing in C++, you might not give variables much thought. They seem to generally work as they do in C++. However, they’re not the same. Whereas in C++ you use variables to reference values, in Python you use names.

First, let’s back up a bit and take a broader look at Python’s object model.

In Python, everything is an object. Numbers are held in objects. Modules are held in objects. Both the object of a class and the class itself are objects. Functions are also objects:

>>> a_list_object = list()
>>> a_list_object
[]
>>> a_class_object = list
>>> a_class_object
<class 'list'>
>>> def sayHi(name):
...      print(f'Hello, {name}')
...
>>> a_function_object = sayHi
>>> a_function_object
<function sayHi at 0x7faa326ac048>

Calling list() creates a new list object, which you assign to a_list_object. Using the name of the class list by itself places a label on the class object. You can place a new label on a function as well. This is a powerful tool and, like all powerful tools, it can be dangerous. (I’m looking at you, Mr. Chainsaw.)

Moving back to the Python vs C++ discussion, note that this is behavior is different from what you’ll see in C++. Unlike Python, C++ has variables that are assigned to a memory location, and you must indicate how much memory that variable will use:

int an_int;
float a_big_array_of_floats[REALLY_BIG_NUMBER];

In Python, all objects are created in memory, and you apply labels to them. The labels themselves don’t have types, and they can be put on any type of object:

>>> my_flexible_name = 1
>>> my_flexible_name
1
>>> my_flexible_name = 'This is a string'
>>> my_flexible_name
'This is a string'
>>> my_flexible_name = [3, 'more info', 3.26]
>>> my_flexible_name
[3, 'more info', 3.26]
>>> my_flexible_name = print
>>> my_flexible_name
<built-in function print>

You can assign my_flexible_name to any type of object, and Python will just roll with it.

When you’re comparing Python vs C++, the difference in variables vs names can be a bit confusing, but it comes with some excellent benefits. One is that in Python you don’t have pointers, and you never need to think about heap vs stack issues. You’ll dive into memory management a bit later in this article.

Comprehensions

Python has a language feature called list comprehensions. While it’s possible to emulate list comprehensions in C++, it’s fairly tricky. In Python, they’re a basic tool that’s taught to beginning programmers.

One way of thinking about list comprehensions is that they’re like a super-charged initializer for lists, dicts, or sets. Given one iterable object, you can create a list, and filter or modify the original as you do so:

>>> [x**2 for x in range(5)]
[0, 1, 4, 9, 16]

This script starts with the iterable range(5) and creates a list that contains the square for each item in the iterable.

It’s possible to add conditions to the values in the first iterable:

>>> odd_squares = [x**2 for x in range(5) if x % 2]
>>> odd_squares
[1, 9]

The if x % 2 at the end of this comprehension limits the numbers used from range(5) to only the odd ones.

At this point you might be having two thoughts:

1. That’s a powerful syntax trick that will simplify some parts of my code.
2. You can do the same thing in C++.

While it’s true that you can create a vector of the squares of the odd numbers in C++, doing so usually means a little more code:

std::vector<int> odd_squares;
for (int ii = 0; ii < 10; ++ii) {
    if (ii % 2) {
        odd_squares.push_back(ii*ii);
    }
}

For developers coming from C-style languages, list comprehensions are one of the first noticeable ways they can write more Pythonic code. Many developers start writing Python with C++ structure:

odd_squares = []
for ii in range(5):
    if (ii % 2):
        odd_squares.append(ii)

This is perfectly valid Python. It will likely run more slowly, however, and it’s not as clear and concise as the list comprehension. Learning to use list comprehensions will not only speed up your code, but it will also make your code more Pythonic and easier to read!

Python’s std::algorithms

C++ has a rich set of algorithms built into the standard library. Python has a similar set of built-in functions that cover the same ground.

The first and most powerful of these is the in operator, which provides a quite readable test to see if an item is included in a list, set, or dictionary:

>>>