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What does {!r} mean in Python?

What does (slice (None),None) mean in Python?

  • I was reading some numpy codes when I found this: tup = (slice(None),None) X -= X.mean(1) [tup] where X is a numpy array. Could anyone explain this line of code? Especially what the (slice(None), None) part? The python documentation has explanation of what slice() does, but I was unable to find what slice(None) means.

  • Answer:

    All the answers are correct but what's confusing might be how tup=(slice(None), None) translates to the "standard" way of indexing. Basically the code in the question is equivalent to X -= X.mean(axis=1)[::, None] which is also equivalent to X -= X.mean(axis=1)[::, np.newaxis] So slice(None) is adding "::", and the None as the last element in tup is adding a new axis, to transpose the result of mean(axis=1).

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slice(None) creates a slice instance as slice(None, None, None) with default for start, step and end (see [1]). Wrt your code snippet, the tuple returns the individual computed means on the second axis as per [2]. Let's confirm this: >>> import numpy as np >>> arr = np.array([[1, 2], [3, 4]]) >>> arr.mean(1)[(slice(None), None)] array([[ 1.5], [ 3.5]]) >>> arr -= arr.mean(1)[(slice(None), None)] >>> arr array([[0, 0], [0, 0]]) [1] http://docs.python.org/2/library/functions.html#slice [2] http://docs.scipy.org/doc/numpy/reference/generated/numpy.mean.html

Anuj Bhatt

slice() creates an instance of type slice. A slice instance I can be used to index a sequence S in an expression of the form S[I]. Here is the general form: slice(start, limit, step) The result is a slice that is equivalent to start:limit:step. Use None to get the default value for any of the three arguments. that code set up a tuple with (start:limit:step, None)

Paolo Gambardella

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