What Makes The Join() Method Blazingly Faster Than Iteration?

There are two popular ways to concatenate multiple strings:

1. Iterating and appending them to a single string.

2. Using Python’s in-built join() method.

But as shown above, the 2nd approach is significantly faster than the 1st approach.

Here’s why (or maybe stop reading here and try to guess before you read ahead).

When iterating, Python naively executes the instructions it comes across.

Thus, it does not know (beforehand):

• number of strings it will concatenate

• number of white spaces it will need

In other words, iteration inhibits the scope for optimization.

As a result, at every iteration, Python asks for a memory allocation of:

• the string at the current iteration

• the white space added as a separator

This leads to repeated calls to memory. To be precise, the number of calls, in this case, is two times the size of the list.

However, with join(), Python precisely knows (beforehand):

• number of strings it will be concatenating

• number of white spaces it will need

All these are applied for allocation in a single call and are available upfront before concatenation.

To summarize:

• with iteration, the number of memory allocation calls is 2x the list's size.

• with join(), the number of memory allocation calls is just one.

This explains the significant difference in their run-time.

This post is also a reminder to always prefer specific methods over a generalized approach. What do you think?

👉 Over to you: What other ways do you commonly use to optimize native Python code?

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Find the code for my tips here: GitHub.

I like to explore, experiment and write about data science concepts and tools. You can read my articles on Medium. Also, you can connect with me on LinkedIn and Twitter.