Examples and techniques of 5.3.1.1 Chainmap

Here are some scenarios and tips for using the Chainmap class:

Simulates python 's internal seizure of named variables in a chain of mappings:

#python 3.4

Import Builtins

Import Collections

Pylookup = collections. Chainmap (Locals (), Globals (), VARs (Builtins))

Print (Pylookup)

In this example, the local scope naming, global scope naming, and built-in scope naming are chained together, the lookup will be local, then global, and finally the built-in scope.

Let the command line parameters take precedence over the operating system environment variable parameter, and the operating system environment variable parameter takes precedence over the default value example:

#python 3.4

Import Collections

Import OS, Argparse

Defaults = {' Color ': ' Red ', ' user ': ' Guest '}

Parser = Argparse. Argumentparser ()

Parser.add_argument ('-u ', '--user ')

Parser.add_argument ('-C ', '--color ')

namespace = Parser.parse_args ()

Command_line_args = {k:v for k, V in VARs (namespace). Items () if v}

Combined = collections. Chainmap (Command_line_args, Os.environ, defaults)

Print (combined[' color ')

Print (combined[' user ')

The resulting output is as follows:

Red

Guest

In this case, be aware of this line of code:

Collections. Chainmap (Command_line_args, Os.environ, defaults)

The front dictionary is the first to search, the last is the last search.

Cai Junsheng qq:9073204 Shenzhen

Examples and techniques of 5.3.1.1 Chainmap