Tautologies are overrated

Posted by Swaroop C H on March 10, 2005 Filed under: python, tech

[code] $ python2.4

True,False = False, True True False False True a = True if a: ... print 'Hello World' ... if not a: ... print 'Hello World' ... Hello World [/code]

I am a big-time Python fan but this is crazy!

(discovered via python-list)


john says:

A similar problem exists for keywords:

>>> abs(-1)
>>> abs=1
>>> abs(-1)
Traceback (most recent call last):
File "", line 1, in ?
TypeError: 'int' object is not callable

Contrast with Ruby

irb(main):001:0> -1.abs
=> 1
irb(main):002:0> -1.abs =3
NoMethodError: undefined method `abs=' for -1:Fixnum
from (irb):2

Swaroop C H says:

John: Thanks for pointing that out. Looks like there are still many warts in Python.

Pramod Biligiri says:

I tried this in C++: true = false
And the compiler says, "error: non-lvalue in assignment"
I wonder how that works in Python.

Aggelos says:

Wow. Why can you do that? Assign values to False and True?

srid says:

This is not crazy. You have assigned the 'False' bool type to 'a', and you get what you expected. :-)

Actually, python must deny the use of True and False as l-values.

Swaroop C H says:

Pramod: C++ has true and false as special keywords from the beginning. Python just added it (relatively) recently and apparently things like these are still possible.

Swaroop C H says:

Aggelos: Yes, that actually works :| , I guess they have `True` and `False` as just instances of the `bool` class and hence things like these are possible i.e. they are not keywords, they are just objects which can be manipulated like any other.

Swaroop C H says:

Sridhar: I know what the program is doing. I expected it not to allow assignment to `True` and `False`, that is what I am surprised about.

Anand says:

True and False are not constants in Python. In fact they are the only possible instances of the new 'bool' type.

>>> c = bool(5)
>>> c
>>> c = bool(0)
>>> c

However Python should be setting these names as immutable, similar to the None keyword in 2.4

Noorul says:

This should not happen. may be u can suggest python guys not to allow True or False as lvalue.

Bob Ippolito says:

Anything that is not a keyword can be overrided, and there are very few keywords on purpose! It's not a wart, it's a design decision. Live with it :)

Python is for consenting adults.

Aggelos says:

This has a lot of potential for obfuscated stuff. :)

Sidharth Kuruvila says:

john compared ruby properties with python names
Not a good comparision

I think this is closer to what john demonstrated in ruby
using python properties

>>>class absInt(int):
... abs = property(fget=lambda self: abs(self))
>>> i = absInt(-1)
>>> i.abs
>>> i.abs = 5
Traceback (most recent call last):
File "", line 1, in ?
AttributeError: can't set attribute

I think i like the idea of limiting the number of keywords the way python does.


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