选题[tech]: 20210625 Use Python to parse configuration files

sources/tech/20210625 Use Python to parse configuration files.md

sources/tech/20210625 Use Python to parse configuration files.md

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+[#]: subject: (Use Python to parse configuration files)
+[#]: via: (https://opensource.com/article/21/6/parse-configuration-files-python)
+[#]: author: (Moshe Zadka https://opensource.com/users/moshez)
+[#]: collector: (lujun9972)
+[#]: translator: ( )
+[#]: reviewer: ( )
+[#]: publisher: ( )
+[#]: url: ( )
+
+Use Python to parse configuration files
+======
+The first step is choosing a configuration format: INI, JSON, YAML, or
+TOML.
+![Python programming language logo with question marks][1]
+
+Sometimes, a program needs enough parameters that putting them all as command-line arguments or environment variables is not pleasant nor feasible. In those cases, you will want to use a configuration file.
+
+There are several popular formats for configuration files. Among them are the venerable (although occasionally under-defined) `INI` format, the popular but sometimes hard to write by hand `JSON` format, the extensive yet occasionally surprising in details `YAML` format, and the newest addition, `TOML`, which many people have not heard of yet.
+
+Your first task is to choose a format and then to document that choice. With this easy part out of the way, it is time to parse the configuration.
+
+It is sometimes a good idea to have a class that corresponds to the "abstract" data in the configuration. Because this code will do nothing with the configuration, this is the simplest way to show parsing logic.
+
+Imagine the configuration for a file processor: it includes an input directory, an output directory, and which files to pick up.
+
+The abstract definition for the configuration class might look something like:
+
+
+```
+`from __future__ import annotations`[/code] [code]
+
+import attr
+
+@attr.frozen
+class Configuration:
+    @attr.frozen
+    class Files:
+        input_dir: str
+        output_dir: str
+    files: Files
+    @attr.frozen
+    class Parameters:
+        patterns: List[str]
+    parameters: Parameters
+```
+
+To make the format-specific code simpler, you will also write a function to parse this class out of dictionaries. Note that this assumes the configuration will use dashes, not underscores. This kind of discrepancy is not uncommon.
+
+
+```
+def configuration_from_dict(details):
+    files = Configuration.Files(
+        input_dir=details["files"]["input-dir"],
+        output_dir=details["files"]["output-dir"],
+    )
+    parameters = Configuration.Paraneters(
+        patterns=details["parameters"]["patterns"]
+    )
+    return Configuration(
+        files=files,
+        parameters=parameters,
+    )
+```
+
+### JSON
+
+JSON (JavaScript Object Notation) is a JavaScript-like format.
+
+Here is an example configuration in JSON format:
+
+
+```
+json_config = """
+{
+    "files": {
+        "input-dir": "inputs",
+        "output-dir": "outputs"
+    },
+    "parameters": {
+        "patterns": [
+            "*.txt",
+            "*.md"
+        ]
+    }
+}
+"""
+```
+
+The parsing logic parses the JSON into Python's built-in data structures (dictionaries, lists, strings) using the `json` module and then creates the class from the dictionary:
+
+
+```
+import json
+def configuration_from_json(data):
+    parsed = json.loads(data)
+    return configuration_from_dict(parsed)
+```
+
+### INI
+
+The INI format, originally popular on Windows, became a de facto configuration standard.
+
+Here is the same configuration as an INI:
+
+
+```
+ini_config="""
+[files]
+input-dir = inputs
+output-dir = outputs
+
+[parameters]
+patterns = ['*.txt', '*.md']
+"""
+```
+
+Python can parse it using the built-in `configparser` module. The parser behaves as a `dict`-like object, so it can be passed directly to `configuration_from_dict`:
+
+
+```
+import configparser
+
+def configuration_from_ini(data):
+    parser = configparser.ConfigParser()
+    parser.read_string(data)
+    return configuration_from_dict(parser)
+```
+
+### YAML
+
+YAML (Yet Another Markup Language) is an extension of JSON that is designed to be easier to write by hand. It accomplishes this, in part, by having a long specification.
+
+Here is the same configuration in YAML:
+
+
+```
+yaml_config = """
+files:
+  input-dir: inputs
+  output-dir: outputs
+parameters:
+  patterns:
+  - '*.txt'
+  - '*.md'
+"""
+```
+
+For Python to parse this, you will need to install a third-party module. The most popular is `PyYAML` (`pip install pyyaml`). The YAML parser also returns built-in Python data types that can be passed to `configuration_from_dict`. However, the YAML parser expects a stream, so you need to convert the string into a stream.
+
+
+```
+import io
+import yaml
+def configuration_from_yaml(data):
+    fp = io.StringIO(data)
+    parsed = yaml.safe_load(fp)
+    return configuration_from_dict(parsed)
+```
+
+### TOML
+
+TOML (Tom's Own Markup Language) is designed to be a lightweight alternative to YAML. The specification is shorter, and it is already popular in some places (for example, Rust's package manager, Cargo, uses it for package configuration).
+
+Here is the same configuration as a TOML:
+
+
+```
+toml_config = """
+[files]
+input-dir = "inputs"
+output-dir = "outputs"
+
+[parameters]
+patterns = [ "*.txt", "*.md",]
+"""
+```
+
+In order to parse TOML, you need to install a third-party package. The most popular one is called, simply, `toml`. Like YAML and JSON, it returns basic Python data types.
+
+
+```
+import toml
+def configuration_from_toml(data):
+    parsed = toml.loads(data)
+    return configuration_from_dict(parsed)
+```
+
+### Summary
+
+Choosing a configuration format is a subtle tradeoff. However, once you make the decision, Python can parse most of the popular formats using a handful of lines of code.
+
+--------------------------------------------------------------------------------
+
+via: https://opensource.com/article/21/6/parse-configuration-files-python
+
+作者：[Moshe Zadka][a]
+选题：[lujun9972][b]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]: https://opensource.com/users/moshez
+[b]: https://github.com/lujun9972
+[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/python_programming_question.png?itok=cOeJW-8r (Python programming language logo with question marks)