Python Syntax Soup: x in y

By Xah Lee. Date: . Last updated: .

Python has these operators to check if a key exist in dictionary or list:

There are several issues:

• x not in list is not necessary. It can be done with not (x in list)

• The not is part of not in operator. This means, the word not has context dependent semantics.

• not in is a operator. Using English words as operator is not so good. but now that operator is 2 words, separated by space! So, syntactically, it becomes not distinguishable from statements.

• x in y is subform of the loop form for x in y , and also the List Comprehension syntax [… for x in y] . This is a example of context dependent semantics.

• k in d is itself is not necessary. Python has a has_key() method for dictionary, which is in sync with the 20 other dictionary methods. Method syntax is systematic and also informative of its meaning because of the name. Though, strangely, python decided to deprecate it, and in python 3, it's gone.

This is the state of the affair of syntax soup. When you learn a language, there's little governing principle of what symbol can go where (as in formal language), instead, you learn by rote of what symbols or words can go where in what context means what.

Here is another example.

the python import statement syntax, is a prime example of context dependent semantics. witness:

in the above, the two import name have different meaning.

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Context Dependent Semantics