`1` and `True` yield different result for extended slicing about gmpy HOT 4 CLOSED

The same issue leads to confusing error message for simple slice assignment:

    >>> d = gmpy2.xmpz(0b1000)
    >>> d[1:3] = gmpy2.xmpz(0b111)
    >>> bin(d)
    '0b1110'
    >>> d[1:3] = [True,True]
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: must specify sequence of bits

Expected 

>>> d = numpy.array([0,0,0,1], dtype=bool)
>>> d[1:3] = [1,1,1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: shape mismatch: objects cannot be broadcast to a single shape
>>> d[1:3] = [True,True]
>>> to_bin(d)
'1110'

>>> d = [0,0,0,1]
>>> d[1:3] = [1,1,1]
>>> to_bin(d)
'11110'
>>> d = [0,0,0,1]
>>> d[1:3] = [1,1]
>>> to_bin(d)
'1110'

Original comment by `[email protected]` on 2 Jun 2010 at 11:49

My use of True is an ugly hack. When getting bits from a number, GMP treats 0 
as an 
infinitely long number with all bits set to 0. This makes assigning a slice of 
bits 
to 0 very easy. Unfortunately, 1 is considered an infinitely long number with 
the 
first bit set to 1 and all remaining bits set to 0. From GMP's perspective, -1 
is an 
infinitely strong number with all bits set to 1. I'll remove the special 
handling of 
True. Then the sample code will need to use -1 (not intuitive either) or I need 
to 
introduce a bit_mask(n) function to return a number with n bits set. This would 
probably be more useful anyway.

I haven't thought about supporting assignment from a list. Since I'm assigning 
to 
bit positions of a number, I thought it was reasonable that the bit positions 
came 
from another number.

The alternative is to only support setting a slice to either all 0 or all 1.

I'm open to suggestions.

Thanks for the feedback.

I changed slice assignments to only accept 0 or 1. It's not as flexible but 
also not
as confusing. I think this is reasonable since gmpy2 isn't intended for bit 
operations. 

Changed to allow getting and setting arbitrary sequences of bits. True and 
False are no longer treated as special values. To provide an as-long-as-needed 
source of 0 bits, use the value 0. To provide an as-long-as-needed source of 1 
bits, use ~0 (or -1). This matches the behavior of GMP and of 2's complement 
integers.

If you want to use a list of boolean values as input, see the new method pack():

>>> gmpy2.pack([True, False, True],1)
mpz(5)