python - converting string to tuple

Question

I need to write a function that takes a string '(1,2,3,4,5),(5,4,3,2,1)' and returns a list of tuples of the 1st and last element of each tuple, [(1,5),(5,1)]. I was thinking:

def f(givenstring):
    a=givenstring.split(',')
    for i in a[0:-1]:
        tuple(int(i[0,-1]))

but here I'm stucked..

Answer 1

You can use ast.literal_eval():

Safely evaluate an expression node or a string containing a Python expression. The string or node provided may only consist of the following Python literal structures: strings, numbers, tuples, lists, dicts, booleans, and None.

This can be used for safely evaluating strings containing Python expressions from untrusted sources without the need to parse the values oneself.

In your example:

from ast import literal_eval
s = '(1,2,3,4,5),(5,4,3,2,1)'

l = literal_eval(s)
print l
# ((1, 2, 3, 4, 5), (5, 4, 3, 2, 1))

print [(x[0], x[-1]) for x in l]
# [(1, 5), (5, 1)]

Answer 2

You may use eval. I think it'll be the shortest one.

>>> s = '(1,2,3,4,5),(5,4,3,2,1)'
>>> ts = eval(s)
>>> ts
((1, 2, 3, 4, 5), (5, 4, 3, 2, 1))
>>> tsp = [(el[0],el[-1]) for el in ts]
>>> tsp
[(1, 5), (5, 1)]

Still, it's not a good practice to use eval.

Another option is to parse the string using re module.

>>> a = re.findall('\([^)]*\)',s)
>>> a
['(1,2,3,4,5)', '(5,4,3,2,1)']

Regexp pattern means this:

\( #opening parenthesis
[^)]* #from 0 to infinite symbols different from )
\) #closing parenthesis

.

>>> b = [el.strip('()') for el in a]
>>> b
['1,2,3,4,5', '5,4,3,2,1']
>>> c = [el.split(',') for el in b]
>>> c
[['1', '2', '3', '4', '5'], ['5', '4', '3', '2', '1']]
>>> d = [tuple(int(el2) for el2 in el) for el in c]
>>> d
[(1, 2, 3, 4, 5), (5, 4, 3, 2, 1)]

Also, you may do the following:

>>> [tuple(int(i) for i in el.strip('()').split(',')) for el in s.split('),(')]
[(1, 2, 3, 4, 5), (5, 4, 3, 2, 1)]

This approach takes not modules at all. But it's not very robust (if the input string will have some inconsistency, e.g. space between parentheses and comma ...), (..., then noting will work).

Answer 3

In this case, the ast module could be useful:

>>> from ast import literal_eval
>>> s = '(1,2,3,4,5),(5,4,3,2,1)'
>>> my_tuples = literal_eval(s)
>>> my_tuples
((1, 2, 3, 4, 5), (5, 4, 3, 2, 1))

So, my_tuples has a tuple with the tuples of your string. Now, we can get the first and last element of all your tuples using a list comprehension:

>> new_tuples = [(t[0], t[-1]) for t in my_tuples]
>>> new_tuples
[(1, 5), (5, 1)]