Question

I need to know if a variable in Python is a string or a dict. Is there anything wrong with the following code?

if type(x) == type(str()):     do_something_with_a_string(x) elif type(x) == type(dict()):     do_somethting_with_a_dict(x) else:     raise ValueError 

Update: I accepted avisser’s answer (though I will change my mind if someone explains why isinstance is preferred over type(x) is).

But thanks to nakedfanatic for reminding me that it’s often cleaner to use a dict (as a case statement) than an if/elif/else series.

Let me elaborate on my use case. If a variable is a string, I need to put it in a list. If it’s a dict, I need a list of the unique values. Here’s what I came up with:

def value_list(x):     cases = {str: lambda t: [t],              dict: lambda t: list(set(t.values()))}     try:         return cases[type(x)](x)     except KeyError:         return None 

If isinstance is preferred, how would you write this value_list() function?

Answer 1

What happens if somebody passes a unicode string to your function? Or a class derived from dict? Or a class implementing a dict-like interface? Following code covers first two cases. If you are using Python 2.6 you might want to use collections.Mapping instead of dict as per the ABC PEP.

def value_list(x):     if isinstance(x, dict):         return list(set(x.values()))     elif isinstance(x, basestring):         return [x]     else:         return None