Python Coding Style – Literals vs Instantiating by Name Lists and Dicts

coding-stylepython

In Python, what is the difference in these declarations…

my_list = []
my_list = list()

…and in these?

my_dict = {}
my_dict = dict()

Are they interpreted the same, and why would you use one over the other? I haven't seen or noticed a difference.

Best Answer

The difference between

my_list = list()

and

my_list = []

is that list requires a namespace lookup, first in the module level globals, then in the builtins.

On the other hand, [] is a list literal and is parsed as creating a new list from the language, which doesn't require any name lookups. So the literal is faster on object creation.

Otherwise, they are both the same, for empty lists.

The same analogously applies to dict() and {}.

(Slightly beyond the scope of the question, but the difference, as constructors of non-empty lists, is that list() takes an iterable to construct the list, and [] constructs the list with only the objects with which you create it, usually literals.)

Related Solutions

Java Coding Style – How to Return Multiple Values

How is this normally done in Java?

Painfully.

One option is to use an array, like this Python snippet that returns a list or tuple...

Another option would be to return a hash/dict, like this JavaScript example...

These techniques don't work well in Java; the values would have to be downcast from Object.

One more would be to create a custom object just for this purpose, like this Java example...

This is most common, but it is tedious to create all those little classes. In C++ it is common to use std::pair but this is not commonly done in Java.

With Lombok it is quite easy to create little custom objects:

@RequiredArgsConstructor
public class X {
  private final boolean status;
  private final byte[] data;

}

Python – Cost of Dict and Set In-Built Types

http://wiki.python.org/moin/TimeComplexity

That should pretty much cover everything. What more do you need that's not already on this page?

Best Answer

Related Solutions

Java Coding Style – How to Return Multiple Values

Python – Cost of Dict and Set In-Built Types

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