Difficulty with `cmp', still

[François Pinard]

Hi, people.  I feel a bit opaque, as I cannot quickly get this code to work:


# Return the comparison value of two versions, of two file names with versions.
def compare_versions(a, b):
    a0, a1, a2, a3, a4 = _weights(a)
    b0, b1, b2, b3, b4 = _weights(b)
    print dir(__builtins__)
    cmp = __builtins__.cmp              # because of `import cmp' above
    return cmp(a0, b0) or a1 - b1 or a2 - b2 or a3 - b3 or a4 - b4


When used in batch, I get "AttributeError: cmp".  When used interactively,
everything seems to work fine, and `compare_versions' does what it should.
The `print' statement is a mere try to understanding the problem, and it
has the effect of adding this line to the output, just before the traceback:


['clear', 'copy', 'get', 'has_key', 'items', 'keys', 'update', 'values']


If I do the same `print' interactively, I get (refilled in this message):


['ArithmeticError', 'AssertionError', 'AttributeError', 'EOFError',
'Ellipsis', 'EnvironmentError', 'Exception', 'FloatingPointError',
'IOError', 'ImportError', 'IndexError', 'KeyError', 'KeyboardInterrupt',
'LookupError', 'MemoryError', 'NameError', 'None', 'NotImplementedError',
'OSError', 'OverflowError', 'RuntimeError', 'StandardError', 'SyntaxError',
'SystemError', 'SystemExit', 'TypeError', 'ValueError', 'ZeroDivisionError',
'_', '__debug__', '__doc__', '__import__', '__name__', 'abs', 'apply',
'buffer', 'callable', 'chr', 'cmp', 'coerce', 'compile', 'complex',
'delattr', 'dir', 'divmod', 'eval', 'execfile', 'exit', 'filter', 'float',
'getattr', 'globals', 'hasattr', 'hash', 'hex', 'id', 'input', 'int',
'intern', 'isinstance', 'issubclass', 'len', 'list', 'locals', 'long', 'map',
'max', 'min', 'oct', 'open', 'ord', 'pow', 'quit', 'range', 'raw_input',
'reduce', 'reload', 'repr', 'round', 'setattr', 'slice', 'str', 'tuple',
'type', 'vars', 'xrange']


So far that I know, I'm not playing tricks nor trying to more clever than
Python, I would think my code is rather straight.  I'm surely not trying
to play with the builtin lists, the most I do is trying to get the `cmp'
from it.  I do not see how or why the builtin list get reduced when my code
is executed in batch, and why `cmp' is disappearing.  Would someone have
a suggestion or an idea of what I should study to grasp what is happening?

(By the way, this is not an urgent problem, but it teases me...)

-- 
François Pinard   http://www.iro.umontreal.ca/~pinard