13.10 读取配置文件

## 问题 You want to read configuration files written in the common .ini configuration fileformat. ## 解决方案 The configparser module can be used to read configuration files. For example, supposeyou have this configuration file: ; config.ini; Sample configuration file [installation]library=%(prefix)s/libinclude=%(prefix)s/includebin=%(prefix)s/binprefix=/usr/local # Setting related to debug configuration[debug]log_errors=trueshow_warnings=False [server]port: 8080nworkers: 32pid-file=/tmp/spam.pidroot=/www/rootsignature: > Here is an example of how to read it and extract values: >>> from configparser import ConfigParser >>> cfg = ConfigParser() >>> cfg.read('config.ini') ['config.ini'] >>> cfg.sections() ['installation', 'debug', 'server'] >>> cfg.get('installation','library') '/usr/local/lib' >>> cfg.getboolean('debug','log_errors') True>>> cfg.getint(‘server','port')8080>>> cfg.getint(‘server','nworkers')32>>> print(cfg.get(‘server','signature')) # Brought to you by the Python Cookbook >>> If desired, you can also modify the configuration and write it back to a file using thecfg.write() method. For example: >>> cfg.set('server','port','9000') >>> cfg.set('debug','log_errors','False') >>> import sys >>> cfg.write(sys.stdout) [installation] library = %(prefix)s/lib include = %(prefix)s/include bin = %(prefix)s/bin prefix = /usr/local [debug]log_errors = Falseshow_warnings = False [server]port = 9000nworkers = 32pid-file = /tmp/spam.pidroot = /www/rootsignature = > >>> ## 讨论 Configuration files are well suited as a human-readable format for specifying configu‐ration data to your program. Within each config file, values are grouped into differentsections (e.g., “installation,” “debug,” and “server,” in the example). Each section thenspecifies values for various variables in that section.There are several notable differences between a config file and using a Python sourcefile for the same purpose. First, the syntax is much more permissive and “sloppy.” Forexample, both of these assignments are equivalent: prefix=/usr/localprefix: /usr/local The names used in a config file are also assumed to be case-insensitive. For example: >>> cfg.get('installation','PREFIX') '/usr/local' >>> cfg.get('installation','prefix') '/usr/local' >>> When parsing values, methods such as getboolean() look for any reasonable value.For example, these are all equivalent: > log_errors = truelog_errors = TRUElog_errors = Yeslog_errors = 1 Perhaps the most significant difference between a config file and Python code is that,unlike scripts, configuration files are not executed in a top-down manner. Instead, thefile is read in its entirety. If variable substitutions are made, they are done after the fact.For example, in this part of the config file, it doesn’t matter that the prefix variable isassigned after other variables that happen to use it: > [installation]library=%(prefix)s/libinclude=%(prefix)s/includebin=%(prefix)s/binprefix=/usr/local An easily overlooked feature of ConfigParser is that it can read multiple configurationfiles together and merge their results into a single configuration. For example, supposea user made their own configuration file that looked like this: > > ; ~/.config.ini[installation]prefix=/Users/beazley/test > [debug]log_errors=False This file can be merged with the previous configuration by reading it separately. Forexample: >>> # Previously read configuration >>> cfg.get('installation', 'prefix') '/usr/local' >>> # Merge in user-specific configuration >>> import os >>> cfg.read(os.path.expanduser('~/.config.ini')) ['/Users/beazley/.config.ini'] >>> cfg.get('installation', 'prefix') '/Users/beazley/test' >>> cfg.get('installation', 'library') '/Users/beazley/test/lib' >>> cfg.getboolean('debug', 'log_errors') False >>> Observe how the override of the prefix variable affects other related variables, such asthe setting of library. This works because variable interpolation is performed as lateas possible. You can see this by trying the following experiment: >>> cfg.get('installation','library') '/Users/beazley/test/lib' >>> cfg.set('installation','prefix','/tmp/dir') >>> cfg.get('installation','library') '/tmp/dir/lib' >>> Finally, it’s important to note that Python does not support the full range of features youmight find in an .ini file used by other programs (e.g., applications on Windows). Makesure you consult the configparser documentation for the finer details of the syntaxand supported features.