List Comprehension or Generator Expression with `join()`? Which is More Efficient?

List Comprehension without Brackets: Generator Expressions

In Python, it is possible to create a list of elements using list comprehensions, which have a syntax similar to set comprehensions but with square brackets. However, it is also possible to achieve the same result without using brackets, leading to a construction that resembles a generator expression.

Consider the following example:

''.join([str(_) for _ in range(10)])

This code creates a list of strings representing the numbers from 0 to 9 and joins them into a single string using the join method. Interestingly, the following code produces the same result without using square brackets:

''.join(str(_) for _ in range(10))

The reason for this behavior is that the code without brackets represents a generator expression, which is similar to a list comprehension but returns an iterable object instead of a list. Generator expressions are more memory-efficient and faster than list comprehensions, as they do not create the entire list in memory.

However, in the case of the join method, using a list comprehension is more efficient because join operates more efficiently on actual lists. Therefore, it is generally recommended to use list comprehensions with join for better performance.

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