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版本: 0.10

Variables

In KCL, variables can be defined using assign statements. For example, the following statement defines a variable spam to a string "ham".

spam = "ham"

There are two types of variables, which are global variables and list comprehension local variables.

  • A global variable is defined not within any context.
  • A comprehension local variable is defined inside a comprehension.

A variable can be used after definition, until the end of the current scope.

For a global variable, the scope is the module it is defined in. Note that a module can consists of multiple source files.

For a list comprehension local variable, the scope is the list comprehension it is defined in.

More information on modules, list comprehensions and scopes will be discussed in later chapters.

Immutability​

Global variables are immutable. In other words, once defined such a variable cannot be redefined (or, i.e., modified).

The following code is illegal, and KCL will report an error during evaluation.

spam = "ham"
spam = "eggs" # Error: The immutability rule is violated!
  • A variable starts with the _ character is mutable.
_spam
cond = True
if cond:
    _spam = "ham"
else:
    _spam = "eggs"

Variable Exporting​

As shown in the preview chapter, KCL is able to export evaluation results to the standard output according to a target data format.

The rules are the followings:

  • Living global variables at the end of an evaluation will be dumped out.
  • If the name of a variable starts with the _ character, it will not be dumped out.

Uniqueness of Exported Variable Identifier​

Each exported variable identifier must be unique in its package, so that an exported variable could be located uniquely by package location path and variable identifier, such as 'a.b.c:var', in which 'a.b.c' locates a package.

Two variable identifiers are different if:

  • they are spelled differently
  • they are defined in different packages and are not compiled in a single execution

Identifying an exported variable should be supported by the kcl compiler, which needs to provide corresponding identifying features through the command line and api form.