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Strange side effect from reading a list of numbers from argv

Neel Krishnaswami neelk at brick.cswv.com
Sun Jul 18 19:12:30 EDT 1999 
In article <O3sk3.6047$Sg6.52046 at news1.rdc1.sdca.home.com>,
Harold Gottschalk <heg at softbrew.com> wrote:
>I just wrote this program to become familiar with python and I do not
>understand why it works with a list I create with in the program and not
>when I use sys.argv[1:].
>
>I am sure there is some nuance I am missing, but an explanation to its
>behavior would be helpful.

The elements of sys.argv are *strings*, and strings compare 
ascii-betically:

>>> "3" > "2"
1

>>> "3" > "20"
1

Just like "bat" > "ant", and "bat" > "anthropologist". 

To convert elements of sys.argv (or any string) into numbers, use the
int() or float() built-in functions, depending on whether you want an
integer or floating-point number.

>>> int("3") > int("2")
1

>>> int("3") > int("20")
0

More like what you wanted, right? 

It would be nice if the comparison between your number and the string
had raised an exception, but that's not how Python comparisons work --
the language reference documents this in Section 5.9:

  The operators "<", ">", "==", ">=", "<=", and "!=" compare the values
  of two objects. The objects needn't have the same type. If both are
  numbers, they are coverted to a common type. Otherwise, objects of
  different types always compare unequal, and are ordered consistently
  but arbitrarily.

Nonetheless, this is IMHO a misfeature. We'll just have to live with
a language that's 95% perfect. :)


Neel
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