Python implementation of SHA1
I'm no expert on SHA1, and I see mostly small things, but one big thing that gave me pause. As an overall observation, you seem to value readability over memory usage. Since hashes are often used for large files, this seems to be a risky tradeoff, and python does provide good ways to handle this as well. More of this in the third observation below. On to the observations:


pBits being represented as a series of characters for each bit seems unusual for such low level work. I would typically expect to see this as a hexadecimal string, or even a number. Yet except for possible difficulties comparing it to other implementations that favor less space usage, this seems to make things easy to read.
Filling out pBits to lengths congruent to 448 mod 512 could be done in a single step by figuring out how many bits of padding are necessary and doing a single pBits += "0" * padding; there's no need for a while loop.
This one's important: chunks() should probably be a generator so that you don't have three copies of your data around (one original, one expanded a bit to a byte, and one in chunked strings); correspondingly this could serve out the trailing zero and length bits. Let's examine what this would look like, as 3 copies of large data is really large. Let's remove the sliced copies, creating only one chunk at a time:

def iterchunks(bits, chunkbits=512):
    for i in range(0, len(bits), chunkbits):
        yield bits[i:i + chunkbits]


Note that if the pBits generation was here, checking i + chunkbits against len(bits) and the bytes to bits conversion, you could process a file-like stream instead of a string, taking you further down, from the original 3 copies (with two of them 8 times larger than usual) to not even a full copy of your data in memory at one time.
Back to small ideas. Your bit string could be more literally a bit string, storing \0 and \1 characters. This would let you simplify setting up w into w = map(ord, words). (Of course with the appropriate helper, you could do that with the 0 and 1 characters of your current string.)
I'm not a big fan of the for i in range(0, 80): if 0 &lt;= i &lt;= 19: ... section. It feels like it might read better with a series of for loops that do their different thing then call a helper:

for i in range(0, 19):
    f = (b &amp; c) | ((~b) &amp; d)
    k = 0x5A827999
    a, b, c, d, e = update(a, b, c, d, e, f, k, w[i])
for i in range(20, 39):
    f = b ^ c ^ d
    k = 0x6ED9EBA1
    a, b, c, d, e = update(a, b, c, d, e, f, k, w[i])
...


but that seems pretty ridiculous, and would probably hurt performance to boot. After looking more closely at the obvious alternative, I'm going to withdraw this complaint.