Convert only float in the mixed type list

Question

I have the following list:

ls1 = ['xxx', 3.88884, 2.383, 1.999, '-']

what I want to do is to convert the float value into "%.2f", resulting this:

['xxx', '3.88', '2.38', '1.99', '-']

But why this code failed?

def stringify_result_list(reslist):
    finals = []
    print reslist
    for res in reslist:
        if type(res) == 'float':
           tmp = "%.2f" % res
           finals.append(tmp)
        else:
           finals.append(res)
    print finals
    return finals

 stringify_result_list(ls1)

Answer 1

type(res) does not return what you think it does:

>>> type(1.0)
<type 'float'>
>>> type(type(1.0))
<type 'type'>
>>>

As you can see, it returns the type object for floats, not the string "float". You can read about this behavior in the docs:

class type(object)
class type(name, bases, dict)

With one argument, return the type of an object. The return value is a type object. The isinstance() built-in function is recommended for testing the type of an object.

---

That said, you can greatly simplify your function by using a list comprehension:

def stringify_result_list(reslist):
    return ["%.2f" % x if isinstance(x, float) else x for x in reslist]

You'll notice too that I used isinstance to test whether each item is a float. As the quote above says, this is the preferred method of typechecking in Python.