Defining constant strings

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  • Hans
    #1

    Defining constant strings

    Hi,

    I want to define a couple of constant strings, like in C:
    #define mystring "This is my string"
    or using a const char construction.

    Is this really not possible in Python?

    Hans
    Tags: None
    • Tal Einat
      #2
      Re: Defining constant strings


      Hans wrote:
      Hi,
      >
      I want to define a couple of constant strings, like in C:
      #define mystring "This is my string"
      or using a const char construction.
      >
      Is this really not possible in Python?
      >
      Hans
      It is really not possible.

      The Pythonic way (as far as I have come to know it) is to stop trying
      to protect yourself from yourself, and just don't change something
      which should be constant. Good documentation (including in-code
      comments) and good design are better for preventing programming errors
      than defining things as constant.

      Actually, you can't really define anything as constant in C either. A
      #define can be overriden, a 'const' variable can be casted. These are
      merely conventions which are enforced to some degree by compilers, but
      can easily be worked around.

      There are conventions for constant values in Python too. Usually,
      variables with an all uppercase name are used. As for variables,
      functions, attributes, methods etc. which shouldn't be changed/used
      outside a certain module/class: these are usually prefixed with an
      underscore ("_"). These are not enforced by the interpreter at all,
      though.


      As a side note, Python strings are actually all constants, or
      "immutable" in Python-ish. The variables which reference string objects
      can be changed to reference any other object - that's the nature of
      Python variables. But the strings themselves don't change.

      - Tal

        Comment

        • Tal Einat
          #3
          Re: Defining constant strings

          Hans wrote:
          Hi,
          >
          I want to define a couple of constant strings, like in C:
          #define mystring "This is my string"
          or using a const char construction.
          >
          Is this really not possible in Python?
          >
          Hans
          One last note:

          If you truly insist on having constant variables, you could write a
          class which implements such behavior. You can write a simple class
          which answers your specific needs quite easily.

          Or you could go for a more generic approach, such as this:
          http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/59878


          - Tal

            Comment

            • Peter Otten
              #4
              Re: Defining constant strings

              Hans wrote:
              I want to define a couple of constant strings, like in C:
              #define mystring "This is my string"
              or using a const char construction.
              >
              Is this really not possible in Python?
              No, this is not really not possible in Python:

              $ ls
              preprocess.pyp
              $ cat preprocess.pyp
              #define MYSTRING "Hello, world"
              def f():
              print "Goodbye, " MYOTHERSTRING
              print MYSTRING
              f()
              $ gcc -DMYOTHERSTRING= "'sanity'" -xc -E preprocess.pyp -o preprocess.py
              $ python preprocess.py
              Hello, world
              Goodbye, sanity
              $

              :-)

              Peter

                Comment

                • danielx
                  #5
                  Re: Defining constant strings

                  I would really like to highlight something Tal has already said: Python
                  strings are immutable. That means if you construct a string object, you
                  don't have to worry about someone else going in and changing that
                  object. What might happen, however, is that someone might reassign a
                  variable you have which points to that object. You can tell people not
                  to do this (although you can't force them) using a mechanism Tal has
                  also described. You give your variable a name which is all caps.

                  Hans wrote:
                  Hi,
                  >
                  I want to define a couple of constant strings, like in C:
                  #define mystring "This is my string"
                  or using a const char construction.
                  >
                  Is this really not possible in Python?
                  >
                  Hans
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                  <DIV><FONT face=Arial size=2>Hi,</FONT></DIV>
                  <DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV>
                  <DIV><FONT face=Arial size=2>I want to define a couple of constant strings, like
                  in C:</FONT></DIV>
                  <DIV><FONT face=Arial size=2>#define mystring "This is my string"</FONT></DIV>
                  <DIV><FONT face=Arial size=2>or using a const char construction.</FONT></DIV>
                  <DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV>
                  <DIV><FONT face=Arial size=2>Is this really not possible in Python?</FONT></DIV>
                  <DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV>
                  <DIV><FONT face=Arial size=2>Hans</FONT></DIV></BODY></HTML>
                  >
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