Can't remove item from list in Python - where's the catch?

Question

I'm a newbie so please bear with me:

import random

directions = ['north', 'east', 'south', 'west']
bad_directions = directions[:]

good_direction = random.sample(directions,1)
good_direction = str(good_direction).replace('[','').replace(']','')

bad_directions.remove(good_direction)
print bad_directions

This raises ValueError:

Traceback (most recent call last):
  File "directions.py", line 9, in <module>
    bad_directions.remove(good_direction)
ValueError: list.remove(x): x not in list

I tried checking the types of "good_direction" and "bad_directions[1]" just to see if they're the same and they're both string.

What are you trying to do to good_direction? Did you print and see what it it? — Jayanth Koushik
– Jayanth Koushik, Commented Mar 7, 2014 at 10:38

James Sapam · Accepted Answer · 2014-03-07 10:44:23Z

2

Please put [0] here :

good_direction = random.sample(directions,1)[0]

answered Mar 7, 2014 at 10:44

James Sapam

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2 Comments

user3056783 Over a year ago

wow this simple line did exactly just what I wanted. so actually by calling [0] index it gets string only right? good_direction is therefore a single item list.

James Sapam Over a year ago

btw please don't choose answer too fast and please don't change too frequently.

Paulo Bu · Accepted Answer · 2014-03-07 10:46:17Z

2

I think this line is overkill and error prone:

good_direction = str(good_direction).replace('[','').replace(']','')

If you want to retrieve the string returned by random.sample(directions,1) you can just do:

good_direction=random.sample(directions,1)[0]

Your code is failing because you're missing to replace something from the retrieved string.

I followed your code and the resulting string after the replacement was "'abc'":

>>>import random
>>>l=['abc' for i in range(10)]
>>>s=random.sample(l,1)
>>>s
['abc']
>>>str(s)
"['abc']"
>>>s1=str(s).replace('[','').replace(']','')
>>>s1
"'abc'"    # auch! the single quotes remain there!
>>>s1 in l
False

edited Mar 7, 2014 at 10:46

answered Mar 7, 2014 at 10:41

Paulo Bu

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3 Comments

James Sapam Over a year ago

he did str on list, thats why.

Paulo Bu Over a year ago

Yep, that's the problem.

user3056783 Over a year ago

Yes because I actually searched different topic on Stack Overflow that was about converting list items to string but now I see where the error is.

durga · Accepted Answer · 2014-03-07 10:46:37Z

good directions per your code returns a different string than whats in the directions list. Below is the output.

>>> good_direction
"'east'"
>>> good_direction
"'east'"
>>> good_direction in directions
False

——— may be the below peice of code will achieve what you are trying to achieve.

>>> good_direction = random.choice(directions)
>>> good_direction
'east'
>>> bad_directions.remove(good_direction)
>>> print bad_directions
['north', 'south', 'west']

Daan Timmer · Accepted Answer · 2014-03-07 11:05:17Z

1

The deepcopy is there to make an exact deepcopy version of directions. I don't know if you need it, I just added it.

https://ideone.com/9D8FZI

# your code goes here
import random
import copy

directions = ['north', 'east', 'south', 'west']

bad_directions = copy.deepcopy(directions)

good_directions = random.choice(directions)
bad_directions.remove(good_directions)

print good_directions,bad_directions

If you don't need the deepcopy, then you also don't need to keep directions the original list. It can then be made easier as follows:

https://ideone.com/C49ziQ

# your code goes here
import random

bad_directions = ['north', 'east', 'south', 'west']

good_directions = random.choice(bad_directions)
bad_directions.remove(good_directions)

print good_directions,bad_directions

answered Mar 7, 2014 at 11:05

Daan Timmer

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4 Comments

snobb Over a year ago

bad_directions = directions[:] <-- this is the deepcopy as well.

Paulo Bu Over a year ago

@Alex Not really, that's a shallow copy: docs.python.org/2/library/copy.html

snobb Over a year ago

@Paolo: I agree, but in case of strings which are immutable, it does not make much difference. I should have used the correct wording though. Thanks for correction. Overall great answer from Daan and my vote belongs here. :)

Daan Timmer Over a year ago

@Alex until someone implements a mutable string, or until he starts using objects instead of strings for the direction... Hence I put in the example of the deep copy instead of shallow copy

Gassa · Accepted Answer · 2014-03-07 10:48:56Z

Here is another way to do that:

import random

directions = ['north', 'east', 'south', 'west']
index = random.randrange (len (directions)) # number 0, 1, 2 or 3
good_direction = directions[index]
bad_directions = directions[:index] + directions[index + 1:] # all but directions[index]
print good_direction
print bad_directions

An example output is:

south
['north', 'east', 'west']

You can discard the lines for finding good_direction if you need only bad_directions.

snobb · Accepted Answer · 2014-03-07 11:02:21Z

0

You can do that differently:

bad_directions.pop(random.randrange(len(bad_directions)))

or

del(bad_directions[random.randrange(len(bad_directions))])

I agree with previous post - it does look like a massive overkill to convert list to string then normalize and then work with it.

edited Mar 7, 2014 at 11:02

answered Mar 7, 2014 at 10:54

snobb

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