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Declares one flag as key to the current module.

Key flags are flags that are deemed really important for a module. They are important when listing help messages; e.g., if the --helpshort command-line flag is used, then only the key flags of the main module are listed (instead of all flags, as in the case of --helpfull).

Sample usage:


flag_name str, the name of an already declared flag. (Redeclaring flags as key, including flags implicitly key because they were declared in this module, is a no-op.)
flag_values FlagValues, the FlagValues instance in which the flag will be declared as a key flag. This should almost never need to be overridden.

ValueError Raised if flag_name not defined as a Python flag.