Python 3.x's dictionary view objects and matplotlib

Question

In python 3.x keys(), values() and items() return views. Now while views certainly have advantages, they also seem to cause some compa

Answer 1

More of that error:

--> 512     return array(a, dtype, copy=False, order=order, subok=True)
    513 
    514 def ascontiguousarray(a, dtype=None):

TypeError: float() argument must be a string or a number, not 'dict_values'

So the minimal example is:

np.array(d.keys(),dtype=float)

Without the dtype specification

In [16]: np.array(d.keys())
Out[16]: array(dict_keys([1, 3]), dtype=object)

The dict_keys is treated as an object. Usually you have to work at keeping np.array from treating an object as a list of numbers.

In [17]: np.fromiter(d.keys(),dtype=float)
Out[17]: array([ 1.,  3.])

np.fromiter can handle d.keys(), treating it as a iterable. So there's some detail in how fromiter handles an iterable that is different from np.array.

A generator expression works the same way, e.g. (i for i in range(4)). fromiter can iterate through it, array either treats it as an object or raises an error.

If all the errors that the SO mentioned boiled down to np.array(...) handling a generator, then it might be possible to fix the behavior with one numpy change. Developers certainly wouldn't want to tweak every function and method that might accept a list. But it feels like a fundamental change that would have to be thoroughly tested. And even then it's likely to produce backward compatibility issues.

The accepted fix, for some time now, has been to pass your code through 2to3.

https://docs.python.org/2/library/2to3.html

for dictionaries:

Fixes dictionary iteration methods. dict.iteritems() is converted to dict.items(), dict.iterkeys() to dict.keys(), and dict.itervalues() to dict.values(). Similarly, dict.viewitems(), dict.viewkeys() and dict.viewvalues() are converted respectively to dict.items(), dict.keys() and dict.values(). It also wraps existing usages of dict.items(), dict.keys(), and dict.values() in a call to list.