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- Differences between keywords and constants in Python
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Yesterday I wrote about the challenges of having true constants in Python and said that there are other mechanisms that achieve basically the same results if all we care about are a few builtin values like True , False , and None.
The most straightforward way is what was done with None in Python 2 and eventually done with True and False in Python 3, which is to make them into keywords.
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This raises the obvious question, namely why the Python people waited until Python 3 to make this change. The following identifiers are used as reserved words, or keywords of the language, and cannot be used as ordinary identifiers.
A keyword cannot be used as an identifier in any context, not merely as a variable whether global to a module or even local to a function. If you try harder, you can nominally create the instance attribute either by directly setting it in self.
Установка котла отопления в частном доме своими руками
True in any context gets you a syntax error. Our hypothetical true constants would not be so restricted. Given this, we can look at some issues with making True and False into keywords in Python 2. If they were and if some version of Python 2 made True and False into keywords, that code would immediately fail to even start running. A later version changed the values to be the result of boolean comparisons.
Differences between keywords and constants in Python
If True and False had been created as keywords, there would be no way to use them and be backwards compatible with versions of Python 2 before they were defined. This is counterproductive, to put it one way.
Python 3 can make this sort of change because Python 3 was already making incompatible changes; in fact, making incompatible changes is its entire point. Python 2 was not in a good position to do it.
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