getting only positive number from a list that containing heterogeneous data type item in python 3

Question

I have a list of items with heterogeneous data types contained in strings.

lst=['1','err','-1',' ','155']

From that, I would like to obtain a new list with only the positive numbers: new_lst=[1,155]

I have tried to avoid negative numbers as shown below. However, I am not able to avoid the strings and empty strings:

lst1=int(lst)
for i in lst1:
    if i<0:
    print i
    else:
    continue

Traceback (most recent call last):
  File "C:/Users/Dev/Documents/Assignment-2/test-2.py", line 22, in <module>
    c3=int(row[3])
ValueError: invalid literal for int() with base 10: 'err'
>>> 

Answer 1

lst=['1','err','-1',' ','155']
newlst = []
for i in lst:
    try:
        if int(i) >= 0:
            newlst.append(int(i))
    except ValueError:
        pass

Answer 2

lst1=int(lst) is the code that is raising the exception. This is essentially trying to cast a list into an int (which you cannot do).

What you want to do is iterate through lst, and for each element, determine if the element can be cast into an int, then check if the element (as an int) is positive.

---

You could do something like this:

lst = ['1','err','-1',' ','155']
new_lst = []

for i in lst:
    try:
        int_i = int(i)           # try to cast i to an int
        if int_i > 0:
            new_lst.append(int_i)
    except ValueError:           # ValueError is raised, i cannot be cast into an int
        pass                     # continue to the next element

---

EDIT: Added ValueError for completeness, see @BigZ's answer.

Answer 3

Here is a one liner that tests if an element is actually a number. If this check isn't done, int(x) will not work and throw and exception.

lst=['1','err','-1',' ','155']
lst1 = filter(None,[(lambda x: x if x.isdigit() and int(x) > 0 else None) (x) for x in lst])

Answer 4

If you just have ints and considering you only want to keep positive numbers str.isdigit is sufficient as you don't care if "-1" fails on str.isdigit:

lst[:] = (ele for ele in lst if ele.isdigit() and int(ele) > 0)

If you did possibly have floats then all the answers using int and a try/except would also fail, if you want to cover "1.0" etc.. you would need to cast to float in the try.