13.10 读取配置文件

问题¶

You want to read configuration files written in the common .ini configuration file format.

解决方案¶

The configparser module can be used to read configuration files. For example, suppose you have this configuration file:

; config.ini ; Sample configuration file

[installation] library=%(prefix)s/lib include=%(prefix)s/include bin=%(prefix)s/bin prefix=/usr/local

# Setting related to debug configuration [debug] log_errors=true show_warnings=False

[server] port: 8080 nworkers: 32 pid-file=/tmp/spam.pid root=/www/root signature:

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’))

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 different sections (e.g., “installation,” “debug,” and “server,” in the example). Each section then specifies values for various variables in that section. There are several notable differences between a config file and using a Python source file for the same purpose. First, the syntax is much more permissive and “sloppy.” For example, both of these assignments are equivalent:

prefix=/usr/local prefix: /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 = true log_errors = TRUE log_errors = Yes log_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, the file 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 is assigned after other variables that happen to use it:

[installation] library=%(prefix)s/lib include=%(prefix)s/include bin=%(prefix)s/bin prefix=/usr/local

An easily overlooked feature of ConfigParser is that it can read multiple configuration files together and merge their results into a single configuration. For example, suppose a 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. For example:

>>> # 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 as the setting of library. This works because variable interpolation is performed as late as 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 you might find in an .ini file used by other programs (e.g., applications on Windows). Make sure you consult the configparser documentation for the finer details of the syntax and supported features.