4 Writing Structured Programs By now you will have a sense of the capabilities of the Python programming language for processing natural language. However, if you’programming when to use functions new to Python or to programming, you may still be wrestling with Python and not feel like you are in full control yet.
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How can you write well-structured, readable programs that you and others will be able to re-use easily? How do the fundamental building blocks work, such as loops, functions and assignment? What are some of the pitfalls with Python programming and how can you avoid them? Along the way, you will consolidate your knowledge of fundamental programming constructs, learn more about using features of the Python language in a natural and concise way, and learn some useful techniques in visualizing natural language data. In the other chapters of this book, we have organized the programming concepts as dictated by the needs of NLP. Here we revert to a more conventional approach where the material is more closely tied to the structure of the programming language.
There’s not room for a complete presentation of the language, so we’ll just focus on the language constructs and idioms that are most important for NLP. However, there are some surprising subtleties here. However, assignment statements do not always involve making copies in this way. Assignment always copies the value of an expression, but a value is not always what you might expect it to be. To understand what is going on here, we need to know how lists are stored in the computer’s memory. Observe that changing one of the items inside our nested list of lists changed them all.