How does "&" work in comparing two type of data in this code?

Question

I am reading Python Cookbook: "1.17. Extracting a Subset of a Dictionary". I got confused with the "&" usage in one piece of the below code example. Who may help elaborate on it a bit?

How does prices.keys() & tech_names work here?

prices = {
    'ACME': 45.23,
    'AAPL': 612.78,
    'IBM': 205.55,
    'HPQ': 37.20,
    'FB': 10.75
}
tech_names = {'AAPL', 'IBM', 'HPQ', 'MSFT'}
p2 = {key: prices[key] for key in prices.keys() & tech_names}

Asked By: Xiaokuan Wei

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Source

Answer 1

The & operator is used to create the intersection of the sets of keys in prices and the values in tech_names.

See the section on Dictionary view objects:

Keys views are set-like since their entries are unique and hashable. […] For set-like views, all of the operations defined for the abstract base class collections.abc.Set are available (for example, ==, <, or ^).

Also see the intersection methods section for set:

 set & other & ...
Return a new set with elements common to the set and all others.

So, the code only defines the prices for the symbols listed in the tech_names set.

Another way of spelling this would be to use a if filter in the dictionary comprehension; this has the advantage that you can then use prices.items() and access the symbol and the price at the same time:

p2 = {symbol: price for symbol, price in prices.values() if symbol in tech_names}

Answered By: Martijn Pieters

How does "&" work in comparing two type of data in this code?

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