Cross-version portability in Python

Question

This question offers two approaches to computing bit length of an int:

All versions: len(bin(x))-2
2.7+: x.bit_length()

I need my code to work with 2.6+, so I cannot use the second solution.

Is this the best I can do?

import sys
if sys.hexversion < 0x02070000
    def bitlen(x): return len(bin(x))-2
else:
    def bitlen(x): return x.bit_length()

The alternative is

try: 
    (1).bit_length()
    def bitlen(x): return x.bit_length()
except AttributeError:
    def bitlen(x): return len(bin(x))-2

jonrsharpe · Accepted Answer · 2015-03-18 16:55:48Z

1

I would probably write it as:

def bitlen(x): 
    """The bit length of the integer x."""
    try:
        return x.bit_length()
    except AttributeError:  # I guess!
        return len(bit(x)) - 2

This removes the need for an explicit version check. If you want to factor the check out of the function, perhaps:

if hasattr(1, 'bit_length'):
    bitlen = lambda x: x.bit_length()
else:
    bitlen = lambda x: len(bit(x)) - 2

edited Mar 18, 2015 at 16:55

answered Mar 18, 2015 at 16:48

jonrsharpe

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6 Comments

sds Over a year ago

the problem with this approach is that the check is done on each invocation

jonrsharpe Over a year ago

@sds is that a problem?

sds Over a year ago

this is definitely an aesthetic problem for me; it could also be a performance issue.

jonrsharpe Over a year ago

@sds updated with an alternative; testing with timeit suggests that the bit_length version is faster, so I'd go with that where possible.

sds Over a year ago

thanks, any reason to use lambda instead of def (IOW, is there a problem with non-top-level def?)

|

George Padolsey · Accepted Answer · 2015-03-18 17:07:04Z

0

This would work on 2.6 and below as well as above.

def bitlen(int_type):
    length = 0
    while (int_type):
        int_type >>= 1
        length += 1
    return(length)
# and then
bitlen(0b10000) # --> 5

Check out, https://wiki.python.org/moin/BitManipulation, for further reading

answered Mar 18, 2015 at 17:07

George Padolsey

139 bronze badges

2 Comments

jonrsharpe Over a year ago

Although portable, this is pretty slow; it takes four times as long as the len version and seven times as long as using bit_length.

George Padolsey Over a year ago

Yes, though I thought to add it in just for plain portablitity :)

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