Re: In precision typing we trust

["Lawrence D’Oliveiro" <ldo@nz.invalid>]

On Fri, 29 Aug 2025 09:33:27 -0000 (UTC), Kevin Chadwick wrote:

> ... I don't believe you understand what I'm talking about as I
> believe Python does not have the ability for easily creating e.g.
> Unsigned_2, Unsigned_4 or Unsigned_15 and writing just those bits by
> assignment to wherever the programmer wants.

Something like the following? (Details of the BitVector class
omitted, but I can supply them if you’re interested):

+*In[2]:*+
[source, ipython3]
----
# constructor: v = BitVector(«width»[, «init»])
# «init» defaults to all zeroes if omitted
v1 = BitVector(3, 2)
v2 = BitVector(5)
v1, v2, int(v1), int(v2)
----

+*Out[2]:*+
----(BitVector(3, [False, True, False]),
 BitVector(5, [False, False, False, False, False]),
 2,
 0)----

+*In[3]:*+
[source, ipython3]
----
# access can be single bits, or use standard Python slice notation
v1[0:2], v1[2], v1[1:3], v2[2:5]
----

+*Out[3]:*+
----(BitVector(2, [False, True]),
 False,
 BitVector(2, [True, False]),
 BitVector(3, [False, False, False]))----

+*In[4]:*+
[source, ipython3]
----
v1[1:3] = [False, True]
v2[:] = 13
----

+*In[5]:*+
[source, ipython3]
----
v1, v2, int(v1), int(v2)
----

+*Out[5]:*+
----(BitVector(3, [False, False, True]),
 BitVector(5, [True, False, True, True, False]),
 4,
 13)----

+*In[6]:*+
[source, ipython3]
----
# “+” does concatenation of BitVectors; note little-endian ordering.
v1 + v2, int(v1 + v2)
----

+*Out[6]:*+
----(BitVector(8, [False, False, True, True, False, True, True, False]), 108)----

+*In[7]:*+
[source, ipython3]
----
# shifts produce results with same number of bits
v2, v2 >> 3, v2 << 3, v2[:3] << 3, (v2 + BitVector(3)) << 3
----

+*Out[7]:*+
----(BitVector(5, [True, False, True, True, False]),
 BitVector(5, [True, False, False, False, False]),
 BitVector(5, [False, False, False, True, False]),
 BitVector(3, [False, False, False]),
 BitVector(8, [False, False, False, True, False, True, True, False]))----

+*In[8]:*+
[source, ipython3]
----
v1[1:3]
----

+*Out[8]:*+
----BitVector(2, [False, True])----

Following all raise errors

+*In[9]:*+
[source, ipython3]
----
v2[:1] = v1
----

+*Out[9]:*+
----
    ---------------------------------------------------------------------------

    ValueError                                Traceback (most recent call last)

    Cell In[9], line 1
    ----> 1 v2[:1] = v1

    [traceback details omitted]

    ValueError: value has 3 bits, only allowed 1 for this usage
----

+*In[10]:*+
[source, ipython3]
----
v1[:] = 8
----

+*Out[10]:*+
----
    ---------------------------------------------------------------------------

    ValueError                                Traceback (most recent call last)

    Cell In[10], line 1
    ----> 1 v1[:] = 8

    [traceback details omitted]

    ValueError: value has 4 bits, only allowed 3 for this usage
----

+*In[11]:*+
[source, ipython3]
----
v2[0] = 2
----

+*Out[11]:*+
----
    ---------------------------------------------------------------------------

    ValueError                                Traceback (most recent call last)

    Cell In[11], line 1
    ----> 1 v2[0] = 2

    [traceback details omitted]

    ValueError: value has 2 bits, only allowed 1 for this usage
----

+*In[12]:*+
[source, ipython3]
----
v1[:] = v2[:]
----

+*Out[12]:*+
----
    ---------------------------------------------------------------------------
    ValueError                                Traceback (most recent call last)

    Cell In[12], line 1
    ----> 1 v1[:] = v2[:]

    [traceback details omitted]

    ValueError: value has 5 bits, only allowed 3 for this usage
----