问题
I come from C background and am learning Python. The lack of explicit type-safety is disturbing, but I am getting used to it. The lack of built-in contract-based programming (pure abstract classes, interfaces) is something to get used to, in the face of all the advantages of a dynamic language.
However, the inability to request const-cortectness is driving me crazy! Why are there no constants in Python? Why are class-level constants discouraged?
回答1:
C and Python belongs to two different classes of languages.
The former one is statically typed. The latter is dynamic.
In a statically typed language, the type checker is able to infer the type of each expression and check if this match the given declaration during the "compilation" phase.
In a dynamically typed language, the required type information is not available until run-time. And the type of an expression may vary from one run to an other. Of course, you could add type checking during program execution. This is not the choice made in Python. This has for advantage to allow "duck typing". The drawback is the interpreter is not able to check for type correctness.
Concerning the const keyword. This is a type modifier. Restricting the allowed use of a variable (and sometime modifying allowed compiler optimization). It seems quite inefficient to check that at run-time for a dynamic language. At first analysis, that would imply to check if a variable is const or not for each affectation. This could be optimized, but even so, does it worth the benefit?
Beyond technical aspects, don't forget that each language has its own philosophy. In Python the usual choice is to favor "convention" instead of "restriction". As an example, constant should be spelled in all caps. There is no technical enforcement of that. It is just a convention. If you follow it, your program will behave as expected by "other programmers". If you decide to modify a "constant", Python won't complain. But you should feel like your are doing "something wrong". You break a convention. Maybe you have your reasons for doing so. Maybe you shouldn't have. Your responsibility.
As a final note, in dynamic languages, the "correctness" of a program is much more of the responsibility of your unit testings, than in the hand of the compiler. If you really have difficulties to made the step, you will find around some "code checkers". Those are PyLint, PyChecker, PyFlakes...
回答2:
I don't know why this design decision was made but my personal guess is that there's no explicit const keyword because the key benefits of constants are already available:
Constants are good for documentation purposes. If you see a constant, you know that you can't change it. This is also possible by naming conventions.
Constants are useful for function calls. If you pass a constant as a parameter to a function, you can be sure that it isn't changed. In Python functions are "call-by-value" but since python variables are references you effectively pass a copy of a reference. Inside of the function you can mutate the reference but if you reassign it, the changes do not persist outside of the function scope. Therefore, if you pass a number as a variable, it is actually passed "like" a constant. You can assign a new value to the variable. But outside of the function, you still got the old number
Moreover if there was a const keyword, it would create an asymmetry: variables are declared without keyword but consts are declared with a keyword. The logical consequence would be to create a second keyword named var. This is probably a matter of taste. Personally I prefer the minimalistic approach to variable declarations.
You can probably achieve a little more type safety, if you work with immutable data structures like tuples. Be careful however, the tuple itself can not be modified. But if it contains references to mutable objects, these are still mutable even if they belong to a tuple.
Finally you might want to take a look at this snippet: http://code.activestate.com/recipes/65207-constants-in-python/?in=user-97991 I'm not sure if this is an implementation of "class-level constants". But I thought it might be useful.
来源：https://stackoverflow.com/questions/17291791/why-no-const-in-python