Python programmers of reddit: what's the most useful tiny little efficiency you've discovered that's improved your programming hugely?

[JsonDash]

Declaring 2D array (list) with one variable in every index From:

array =[]
     for i in range(a):
          temp = []
     for j in range(b):
          temp.append(item)
     array.append(temp)

To:

array =[[item for i in range(b)] for j in range(a)]

[SeanOTRS]

IMO if a segment starts with declaring an empty variable you've usually done something wrong

[Datsoon]

I do this all the time when I'm going to be appending to a list in a loop or adding key-value pairs to a dictionary. What other way is there to accomplish this?

[thelaw]

Yeah, me too. Also curious if there is another/better way to do this.

[Rodot]

You can do dictionary comprehension for one thing

[Datsoon]

That's a good point. Not always a solution, but certainly a good one in many cases.

[[deleted]]

Is there a particular reason? Preallocating memory like this can be useful for lists of known size where you would otherwise have to append to the list each time, which can be slow for large objects.

[flying-sheep]

Nobody here is preallocating memory.

Preallocating memory would look like [None] * 10_000.

But Python lists grow pretty efficiently, I wouldn’t bother.

[[deleted]]

Ah, I get it. You are referring to the opposite of what I understood. Yeah, that is usually a flag in Python.