Extract numbers from strings in python

I am new to Python and I have a String, I want to extract the numbers from the string. For example.

str1 = "3158 reviews"
print (re.findall('\d+', str1 ))

Output is ['4', '3']

I want to get 3158 only, as an Integer preferably, not as List.

You can filter the string by digits using str.isdigit method,

>>> int(filter(str.isdigit, str1))
3158