在JSON文件中处理Python程序参数

Question

我刚开始学Python，想请教一下怎么处理存放在json格式文件里的程序参数。目前我做的方式像下面这样，但感觉写得太啰嗦了，而且重复输入同样的字符串（有时候用连字符，有时候用下划线）感觉很幼稚，容易出错，真让人烦…… :-)（我还有很多参数呢！）

#!/usr/bin/env python
import sys
import os
import json                 ## for control file parsing

# control parameters
mpi_nodes = 1
cluster_size = None
initial_cutoff = None
# ...

#process the arguments
if len(sys.argv) != 2:
    raise Exception(
        """Usage:
                run_foo <controls.json>
            Where:
                <control.json> is a dictionary of run parameters
        """
    )

# We expect a .json file with our parameters
controlsFileName = sys.argv[1]
err = ""
err += "" #validateFileArgument(controlsFileName, exists=True)

# read in the control parameters from the .json file
try:
    controls = json.load(open(controlsFileName, "r"))
except:
    err +=  "Could not process the file '" + controlsFileName + "'!\n"

# check each control parameter. The first one is optional
if "mpi-nodes" in controls:
    mpi_nodes = controls["mpi-nodes"]
else:
    mpi_nodes = controls["mpi-nodes"] = 1

if "cluster-size" in controls:
    cluster_size = controls["cluster-size"]
else:
    err += "Missing control definition for \"cluster-size\".\n"

if "initial-cutoff" in controls:
    initial_cutoff = controls["initial-cutoff"]
else:
    err += "Missing control definition for \"initial-cutoff\".\n"

# ...

# Quit if any of these things were not true
if len(err) > 0:
    print err
    exit()

#...

这样做是可以的，但我觉得应该有更好的方法。我必须使用json文件，并且参数名要用连字符，这让我很困扰。有没有什么好主意？

我想找一种更静态的绑定方式。也许这就是极限了。

Answer 1

argparse库很不错，它可以帮你处理大部分的参数解析和验证工作，还能打印出漂亮的帮助信息。

[1] http://docs.python.org/dev/library/argparse.html

我今天下午会快速做个演示，告诉你怎么使用它。

Answer 2

假设你有很多参数需要处理，像这样的方法可能会有效：

def underscore(s):
  return s.replace('-','_')

# parameters with default values
for name, default in (("mpi-nodes", 1),):
  globals()[underscore(name)] = controls.get(name, default)

# mandatory parameters
for name in ("cluster-size", "initial-cutoff"):
  try:
    globals()[underscore(name)] = controls[name]
  except KeyError:
    err += "Missing control definition for %r" % name

与其直接使用全局变量，你也可以让这个过程更清晰一些：

def underscore(s):
  return s.replace('-','_')

settings = {}    
# parameters with default values
for name, default in (("mpi-nodes", 1),):
  settings[underscore(name)] = controls.get(name, default)

# mandatory parameters
for name in ("cluster-size", "initial-cutoff"):
  try:
    settings[underscore(name)] = controls[name]
  except KeyError:
    err += "Missing control definition for %r" % name

# print out err if necessary

mpi_nodes = settings['mpi_nodes']
cluster_size = settings['cluster_size']
initial_cutoff = settings['initial_cutoff']

Answer 3

通常，我们是这样做的。

def get_parameters( some_file_name ):
    source= json.loads( some_file_name )
    return dict(
        mpi_nodes= source.get('mpi-nodes',1),
        cluster_size= source['cluster-size'],
        initial_cutoff = source['initial-cutoff'],
    )

 controlsFileName= sys.argv[1]
 try:
     params = get_parameters( controlsFileName )
 except IOError:
     print "Could not process the file '{0}'!".format( controlsFileName )
     sys.exit( 1 )
 except KeyError, e:
     print "Missing control definition for '{0}'.".format( e.message )
     sys.exit( 2 )

最后，params['mpi_nodes']的值就是mpi_nodes。

如果你想要一个简单的变量，可以这样做：mpi_nodes = params['mpi_nodes']

如果你想要一个命名元组，就把get_parameters改成这样：

def get_parameters( some_file_name ):
    Parameters= namedtuple( 'Parameters', 'mpi_nodes, cluster_size, initial_cutoff' )
    return Parameters( source.get('mpi-nodes',1),
        source['cluster-size'],
        source['initial-cutoff'],
    )

我不知道你是否会觉得这样更好。