max/min function on list with strings and integers

You could filter the non numeric using a generator expression:

arr = [5,3,6,"-",3,"-",4,"-"]
result = min(e for e in arr if isinstance(e, int))
print(result)

Output

3

Here's a way using directly the max and min built-in funtions with a custom key:

arr = [5,3,6,"-",3,"-",4,"-"]
max(arr, key=lambda x: (isinstance(x,int), x))
# 6

And similarly for the min:

min(arr, key=lambda x: (not isinstance(x,int), x))
# 3

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Details

For the min, consider the following list comprehension as an "equivalent" of the transformation applied with the key:

sorted([(not isinstance(x,int), x) for x in arr])

[(False, 3),
 (False, 3),
 (False, 4),
 (False, 5),
 (False, 6),
 (True, '-'),
 (True, '-'),
 (True, '-')]

So the min will be the lowest tuple, i.e. (0,3).

And for the max, the highest will be (1,6):

sorted([(isinstance(x,int), x) for x in arr])

[(False, '-'),
 (False, '-'),
 (False, '-'),
 (True, 3),
 (True, 3),
 (True, 4),
 (True, 5),
 (True, 6)]