how is this a non-sequence?

Question

I'm running a list comprehension of a list of numbers as strings so for example the list looks like this

vals = ['0.13', '324', '0.23432']

and try a list comprehension like this:

best = [x for x in vals > 0.02]

and I get a TypeError: iteration over non-sequence.

Isn't a list a sequence that should be the first thing you should be able to iterate through? What is a sequence?

It's hard to find answers to basic questions I'm finding.

Thanks.

Answer 1

You need to check if each item is greater than '0.02', not whether the sequence is greater.

best = [x for x in vals if x > '0.02']

Your original expression, [x for x in vals > '0.02'] is parsed as [x for x in (vals > '0.02')]. Since vals > '0.02' is a boolean value, and not a sequence, it's not possible to iterate over it.

EDIT: I updated this answer to use the string '0.02' per Joe's suggestion in the comments (thank you). That works in this scenario, but in the event that you really wanted to do a numeric comparison instead of a lexicographic one, you could use:

best = [x for x in vals if float(x) > 0.02]

This converts x to a float so that you are comparing a floating-point number to another floating-point number, probably as intended. The result of the list comprehension will still be a list of strings, since we are collecting [x for ...] and not [float(x) for ...]. Just some food for thought.

Answer 2

You are trying to iterate over vals > 0.02 which is not a sequence. If you are trying to filter to just anything > 0.02 do: [x for x in vals if x > 0.02]

Answer 3

You've also got another problem (apart from the missing if x > 0.02), you're comparing a list of string with a float.

So what you probably want is [x for x in vals if x > '0.02']

I've tested that this will give you the expected behaviour. ['324', '0.23432']

Answer 4

No, vals > 0.02 is not exactly a sequence. Also, comparing strings (contained in vals) would not yield the result you expect. You might want to do:

vals = [0.13, 324.0, 0.23432]
best = [x for x in vals if x > 0.02]

That being said, be sure to take a look at NumPy. It allows you to write your example as:

from numpy import *
vals = asarray([0.13, 324.0, 0.23432])
best = vals[vals > 0.02]

While that might not seem much, it offers a plethora of features and advantages you would not want to miss working with numeric arrays and matrices.