parsing python flat and nested lists/tuples

Question

I'm trying to parse a tuple of the form:

a=(1,2)

or

b=((1,2), (3,4)...)

where for a's case the code would be:

x, y = a

and b would be:

for element in b:
    x, y = element

is there an fast and clean way to accept both forms? This is in a MIDI receive callback (x is a pointer to a function to run, and y is intensity data to be passed to a light).

The right way to do this is to always accept the same level of nesting. So, never a = (1, 2), always a = [(1, 2),]. Tarantula is getting downvoted because this doesn't answer your question, but that doesn't change its correctness, albeit it doesn't seem he meant to point that out :). — Julian
– Julian, Commented Aug 19, 2012 at 4:21

Amber · Accepted Answer · 2012-08-19 04:47:05Z

2

# If your input is in in_seq...
if hasattr(in_seq[0], "__iter__"):
    # b case
else:
    # a case

This basically checks to see if the first element of the input sequence is iterable. If it is, then it's your second case (since a tuple is iterable), if it's not, then it's your first case.

If you know for sure that the inputs will be tuples, then you could use this instead:

if isinstance(in_seq[0], tuple):
    # b case
else:
    # a case

Depending on what you want to do, your handling for the 'a' case could be as simple as bundling the single tuple inside a larger tuple and then calling the same code on it as the 'b' case, e.g...

b_case = (a_case,)

Edit: as pointed out in the comments, a better version might be...

from collections import Iterable
if isinstance(in_seq[0], Iterable):
    # ...

edited Aug 19, 2012 at 4:47

answered Aug 19, 2012 at 4:17

Amber

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4 Comments

Bill Over a year ago

Thanks @Amber, I can't vote you up because it requires >= 15 reputation. I'm playing with hasattr now...

JBernardo Over a year ago

It is bad advice to check for __iter__. Use collections.Iterable.

Julian Over a year ago

It's bad advice to use either.

Bill Over a year ago

What is the reason to use collections? The data is pretty sanitized by the time I'm parsing it. Are there any performance issues?

Tarantula · Accepted Answer · 2012-08-19 04:17:10Z

1

The right way to do that would be:

a = ((1,2),) # note the difference
b = ((1,2), (3,4), ...)

for pointer, intensity in a:
   pass # here you do what you want

answered Aug 19, 2012 at 4:17

Tarantula

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4 Comments

Waleed Khan Over a year ago

I think the question is how to accept a=(1,2), not a=((1,2),).

Bill Over a year ago

I tried that, but it doesn't seem to pass through functions, 'def fn(val) <CR> return val <CR> returns (1,2) if i pass it (1,2), --- this is my first question, how do a <CR> in this reply without posting?

Tarantula Over a year ago

@Bill: what you mean by "pass through functions" ?

Tarantula Over a year ago

You'll need to elaborate a little more on your original question, what did you supposed that a function: 'def fn(val): return val' would return with the parameter '(1,2)' ?

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