Use of command line parameter passing library argparse

author ：18cyl

Time ：2022.4.20

Reference article ：python argparse: Command line parameter details

This paper is mainly a supplement to the above article

One 、 Common parameter combinations

parser = argparse.ArgumentParser(description="Something about the program") #  Basically just description  This field is enough   Or nothing 
#  Basically, I only use long options and optional parameters 
parser.add_argument('--weights', type=str, default='123456', help='initial weights') # belt default String parameter of   
parser.add_argument('--bucket', type=str, default='', help='gsutil bucket')# flag value The format is value, nothing flag It's empty string . Don't allow   alone flag
parser.add_argument('--bucket', nargs='?',const="",default='', help='gsutil bucket')# Accept to  flag  Or nothing flag Are empty strings ,flag value The format is value
parser.add_argument('--epochs', type=int, default=300,help='initial epochs')# belt default Of int Type parameter 

parser.add_argument('--rect', action='store_true', help='rectangular training') # Acceptance is true, Default not received False, Not allowed  flag value This assignment format 
parser.add_argument('--rect', action='store_false', help='rectangular training') # Acceptance is false, Default not received true, Not allowed  flag value This assignment format 
parser.add_argument('--resume', action="store_const",const=5, default=False, help='resume most recent training')#  Accept to --resume Then for 5(const), If not received, it is False(default)  Not allowed  flag value The format of  store_true、store_false yes store_const A special form of 

# nargs ‘？’ Indicates at most one ,‘+’ It means at least one ,‘*’ There can be any , A number indicates that you must specify a number of 
#  When the command line passes through multiple parameters, the format is   flag value1 value2 value3. If not specified type by int And so on, the default is str,
# nargs by ？* Or greater than 1 The accepted results are list. Even if only one number is accepted 
parser.add_argument('--img-size', nargs='+', type=int, default=[640, 640], help='[train, test] image sizes')# belt default The majority of int type  
parser.add_argument('--resume', nargs='?', const=1, default=False, help='resume most recent training') # Received --resume  If there is no belt in the back value Save as 1, If you receive --resume value1 The format is saved as value1  If not received --resume  Then for False

opt = parser.parse_args() # 3、 Using parser object built-in methods , Start parsing parameters , Get parsing result object   Namespace(about='input')

print(opt) 
print(opt.about) # 4、opt To parse the result object   adopt   Instance variables   Get the parameters passed in from the command line 

Two 、 Using frames

There are four steps ：

import argparse

parser = argparse.ArgumentParser() #1、 initialization  ArgumentParser() object   namely ： Get parser object 
parser.add_argument("-a","--about",type=str,help="this is a para named about") #2、 Using parser object built-in methods , Add parameter resolution rules   See the following article for specific rules 
opt = parser.parse_args() # 3、 Using parser object built-in methods , Start parsing parameters , Get parsing result object   Namespace(about='input')

print(opt) 
print(opt.about) # 4、opt To parse the result object   adopt   Instance variables   Get the parameters passed in from the command line  

See here some doubt ： The input is -a Parameters But the result is only about Value ？ In front of the parameter -|– What does that mean? ？ If the hands-on experiment finds print(opt.a) Why the report is wrong ？ Then go on Watch and experiment Well ！ If you have any questions, please feel free to comment in the comments area ！

3、 ... and 、 Instructions

1. perform python test.py -h
   
   Explain each part ： The difference between positional parameters and optional parameters Explain later . The rest usage Field 、Desctiption Field 、 The contents in the supplementary description field can be through

argparse.ArgumentParser() #1、 initialization 

Set when obtaining the resolution object

# usage Field 
usage:  The program name  [-h|--help] .....

# Description Field 
 Program function description 

#  Description of position parameters （ Mandatory ）
positional arguments:
 ...
 
#  Optional parameter description 
optional arguments:
...

#  Additional description fields 
...

Parameter description ：

2. Test use argparse.ArgumentParser()
Show me ：

import argparse

parser = argparse.ArgumentParser(prog="Test.py",usage="Good-usage",description="OK-description",epilog="Yes-epilog") # initialization  ArgumentParser() object   namely ： Get parser object 
parser.add_argument("-a","--about",type=str,help="this is a para named about") # Using parser object built-in methods , Add parameter resolution rules   See the following article for specific rules 
opt = parser.parse_args() #  Using parser object built-in methods , Start parsing parameters , Get parsing result object 

print(opt)
print(opt.about)

So you can see usage Field , If it doesn't change , People will automatically display the parameters of this program , It's better not to add .epilog、description、usage Personal feelings all play an explanatory role . therefore You can just use description Field , Other automatic defaults are fine . as for prog Field , I don't see any bird use .

3. Introduce parser.add_argument() Parameters in
   
4. name And flags The difference between
   My understanding is that No - perhaps – Means name, This parameter is classified as positional argument Positional arguments , With -|– Yes. flags, The parameters in this form are classified as optional argument Optional parameters .flags Is designed for long and short options , See the figure below for details

It should be noted that ：1、 One add_argument() Can't appear at the same time name and flags( Two kinds of flags None of the parameters of the format )
2、positional argument Positional parameters are on the command line They must be assigned in order , Can only be assigned by relative position, not by name=value This form , Readability is extremely unfriendly , Because it must be assigned a value , therefore default The default value is nothing ,required Nor can it be used with position parameters , If one of the parameters uses nargs=“*” perhaps + The representative can accept one or more , After setting the number of variable parameters of zero or more parameters , Extremely unfriendly to use . Pure advice is not to

parser.add_argument("e",type=str,default="123",help="this is a positional argument") #nagrs You can set how many numbers this parameter can accept 
parser.add_argument("f",type=str,default="123",help="this is a positional argument")

3、optional argument Optional parameters can appear in one of the long options or at the same time , It defaults to... Even if you don't enter it None, The input mode is in name value The format of （ There is a space in the middle ） If this parameter receives more than one parameter, use name value1 value2 … The format of How many parameters are received in order to nargs This field controls .

import argparse

parser = argparse.ArgumentParser(description="this is a test about argparse") # initialization  ArgumentParser() object   namely ： Get parser object 
parser.add_argument("-a","--about",type=str,default="123",required=False,help="this is an optional argument named about") # Using parser object built-in methods , Add parameter resolution rules   See the following article for specific rules 
parser.add_argument("-b",type=str,help="this is an optional argument")
parser.add_argument("--c",type=str,help="this is an optional argument")
opt = parser.parse_args() #  Using parser object built-in methods , Start parsing parameters , Get parsing result object 

print(opt)
# print(opt.about)

It is worth noting that Used in the program is opt.about, Not at all opt.–about To get about The value of this , If the long option and the short option appear at the same time ,opt.a（ Short options ） It's a mistake , The passed in parameters can only be obtained through the long option .

4. action Field usage
When the command line receives a message with action Field , Actions performed

More commonly used is store_true And store_false. That is, when receiving the parameter, set the value of the parameter to True perhaps False, If no value is received , The default setting is default Corresponding value , without default The fields are False perhaps True,.
Need to pay attention to when , When using the store_true perhaps store_false When Out of commission flag value The format of , and There can only be one flag（ After all, it's not an assignment operation ） also Cannot be associated with type Fields appear at the same time .

parser.add_argument("-a","--about",action="store_true",required=False,help="this is an optional argument named about") # Using parser object built-in methods , Add parameter resolution rules   See the following article for specific rules 
parser.add_argument("-b",action="store_false",help="this is an optional argument")
parser.add_argument("--c",type=str,help="this is an optional argument")