Lua-style in Python

Question

Variable a doesn't exist

I can write in Lua language:

a = (a or 0) + 1

Now a = 1

this allows me not to declare the variable "a" in advance. This is analog of

a = 0
a = a + 1

How can I do the same in Python?

a = (a or 0) + 1

P.S. Why is it important? To avoid assign zero to variables:

python (4 lines of code):

for ticker in ticker_list:
    total_volume[ticker] = 0

for a in range (1,10):
    total_volume[ticker] = total_volume[ticker] + a

lua (2 lines of code):

for a=1,9 do:
    total_volume[ticker] = (total_volume[ticker] or 0) + a

I suppose a = (None or 0) + 1would do what you want, but I cannot see a use-case. — warped
– warped, Commented Apr 11, 2020 at 10:09
What's the benefit of doing that rather than your first approach of defining a=0 ? — Devesh Kumar Singh
– Devesh Kumar Singh, Commented Apr 11, 2020 at 10:09
@IgorK.: "dear friend it saves many lines of code" It saves exactly one line of code. You could easily declare a local variable in the loop which contains either the value or 0 if the value were none, then add one to the local variable. — Nicol Bolas
– Nicol Bolas, Commented Apr 11, 2020 at 17:10

Nifim · Accepted Answer · 2020-04-13 22:02:54Z

1

Python does have a ternary operator, like Lua and many other languages

But unlike Lua, Python does not handle an undefined variable as a "false" value, it will throw an error if you attempt to evaluate an undefined variable. Specifically in your case you will get a key error for the key ticker not being present.

Lua Ternary:

    total_volume[ticker] = (total_volume[ticker] or 0) + a

Python Equivalent:

    total_volume[ticker] = (total_volume[ticker] if ticker in total_volume else 0) + a

Where Lua will pass you the value before the or if it is truthy, python will pass you the value before the if when the statement that follows is true, and the value after the else when it is false.

To safely evaluate if the key was in the dictionary we can use in.

This is mostly to demonstrate the ternary operation you were using in Lua and how to do it in Python, but that doesn't mean it is the right tool for this problem.

A cleaner solution I would suggest:

   total_volume[ticker] = total_volume.get(ticker, 0) + 1

It is less code and easier to reason about. get will return total_volume[ticker] or if the key is not present in the dictionary it will return the default value, 0.

edited Apr 13, 2020 at 22:02

answered Apr 13, 2020 at 21:28

Nifim

5,0312 gold badges16 silver badges36 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Igor K. Over a year ago

Thank You! Which of them is faster? It is important for me because I use it in millions lines of datafiles.

Gabio · Accepted Answer · 2020-04-11 11:08:14Z

1

For your use case, I would suggest to use defaultdict:

from collections import defaultdict

d = defaultdict(int)
d["counter"] += 5
print(d.items()) # output: dict_items([('counter', 5)])

BUT -

I don't think it's a good practice but you can use locals() or globals() to access variables dynamically in the desired scope:

a = 1
print(locals().get("a")) # ouput:1

c = locals().get("c", 2) + 1 
print(c) # output 3

edited Apr 11, 2020 at 11:08

answered Apr 11, 2020 at 10:45

Gabio

9,5643 gold badges17 silver badges38 bronze badges

1 Comment

Lydia van Dyke Over a year ago

There is also the possibility of doing it with a standard dict: d["counter"] = d.get("counter", 0) + 5

Collectives™ on Stack Overflow

Lua-style in Python

2 Answers 2

1 Comment

BUT -

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

BUT -

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related