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How to define a python constant (quasi) that has dynamic inputs?

For instance:

MY_MESSAGE = 'Dear %s, hello.' # ... name = "jj" print MY_MESSAGE % name

Does python have a function to execute something like my above code?

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4 answers

Yes, exactly as you wrote it. Your code is ok.

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Python really does not have the same concept of "constant" that you will find in C. So you have a pretty good approach.

I would actually argue that this is one example of the fact that “we all agree with adults” does not hold well. Think that the language is awesome, but constant every time will be ... enjoyable.

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The string format operator:% can also do other neat things:

 STRING = "This is %(foo)s, and this is also %(foo)s" print STRING % {"foo": "BAR"} # => This is BAR, and this is also BAR

This is really useful if you will repeat one argument. Of course blows

 FORMAT_STRING % ("BAR", "BAR")

You can also register and format numbers.

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Your code is correct, but I believe there is a preferable way to do this:

 >>> my_message = 'Dear {:s}, hello.' >>> my_message.format('jj') 'Dear jj, hello.'

Preferably, due to a possible future replacement of the % operator with the .format() method.

As mentioned in Mark Lutz Learning Python. Fourth edition, format method:

  • has several functions not found in the % expression,
  • can make substitution values ​​more explicit,
  • trades the operator, perhaps more than the name of the mnemonic method,
  • does not support different syntax for single and multiple substitution cases,

and then in the same book:

... there is some risk that Python developers may refuse the % expression in favor of the format method in the future. In fact, there is a note in the Python 3.0 manual.

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