I want to pass an index slice as an argument to a function:
def myfunction(some_object_from_which_an_array_will_be_made, my_index=[1:5:2,::3]):
my_array = whatever(some_object_from_which_an_array_will_be_made)
return my_array[my_index]
Obviously this will not work, and obviously in this particular case there might be other ways to do this, but supposing I really want to do stuff this way, how can I use a variable to slice a numpy array?
np.lib.index_tricks has a number of functions (and classes) that can streamline indexing. np.s_ is one such function. It is actually an instance of a class that has a __get_item__ method, so it uses the [] notation that you want.
An illustration of its use:
In [249]: np.s_[1:5:2,::3]
Out[249]: (slice(1, 5, 2), slice(None, None, 3))
In [250]: np.arange(2*10*4).reshape(2,10,4)[_]
Out[250]:
array([[[40, 41, 42, 43],
[52, 53, 54, 55],
[64, 65, 66, 67],
[76, 77, 78, 79]]])
In [251]: np.arange(2*10*4).reshape(2,10,4)[1:5:2,::3]
Out[251]:
array([[[40, 41, 42, 43],
[52, 53, 54, 55],
[64, 65, 66, 67],
[76, 77, 78, 79]]])
Notice that it constructs the same tuple of slices that ajcr did.
_ is the temporary variable that IPython uses for the last result.
To pass such a tuple to a function, try:
def myfunction(some_object_from_which_an_array_will_be_made, my_index=np.s_[:,:]):
my_array = whatever(some_object_from_which_an_array_will_be_made)
return my_array[my_index]
I = np.s_[1:5:2,::3]
myfunction(obj, my_index=I)
np.s_var will not work? And even then, declaring var = [2:5] will tell me it's invalid syntax.
var=np.s[2:5]; foo(obj, my_index=var). See my added example.var=np.s_[2:5] in your last comment. Also the way it ended up working was not my_array[_] but rather my_array[var]One way is to build a slice object (or a tuple of slice objects) and pass it in to the function to use as the index.
For example, the index notation
my_array[1:5:2, ::3]
is equivalent to
my_array[slice(1,5,2), slice(None,None,3)]
So your function could become:
def myfunction(some_object, my_index=(slice(1,5,2), slice(None,None,3))):
my_array = whatever(some_object)
return my_array[my_index]