Display help message with python argparse when script is called without any arguments

Question

This might be a simple one  Assume I have a program that uses argparse to process command line arguments options  The following will print the  help  message     myprogram -h   or     myprogram --help   But  if I run the script without any arguments whatsoever  it doesn t do anything  What I want it to do is to display the usage message when it is called with no arguments  How is that done

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There are a pair of one-liners with sys argv 1    a very common Python s idiom to refer the command line arguments  being sys argv 0  the script s name  that can do the job   The first one is self-explanatory  clean and pythonic   args   parser parse args None if sys argv 1   else   -h      The second one is a little hackier  Combining the previously evaluated fact that an empty list is False with the True    1 and False    0 equivalences you get this   args   parser parse args  None    -h    not sys argv 1       Maybe too many brackets  but pretty clear if a previous argument selection was made       av   sys argv args   parser parse args  None    -h    not av

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This answer comes from Steven Bethard on Google groups  I m reposting it here to make it easier for people without a Google account to access   You can override the default behavior of the error method   import argparse import sys  class MyParser argparse ArgumentParser       def error self  message           sys stderr write  error   s n    message          self print help           sys exit 2   parser   MyParser   parser add argument  foo   nargs      args   parser parse args       Note that the above solution will print the help message whenever the error method is triggered  For example  test py --blah will print the help message too if --blah isn t a valid option   If you want to print the help message only if no arguments are supplied on the command line  then perhaps this is still the easiest way   import argparse import sys  parser argparse ArgumentParser   parser add argument  foo   nargs      if len sys argv   1      parser print help sys stderr      sys exit 1  args parser parse args       Note that parser print help   prints to stdout by default  As init js suggests  use parser print help sys stderr  to print to stderr

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This isn t good  also  because intercepts all errors   but   def  error parser       def wrapper interceptor           parser print help            sys exit -1       return wrapper  def  args get args sys argv 1         parser   argparser ArgumentParser        parser error    error parser       parser add argument                Here is definition of the error function of the ArgumentParser class   https   github com python cpython blob 276eb67c29d05a93fbc22eea5470282e73700d20 Lib argparse py L2374    As you see  following signature  it takes two arguments  However  functions outside the class nothing knows about first argument  self  because  roughly speaking  this is parameter for the class   I know  that you know     Thereby  just pass own self and message in  error      can t    def  error self  message       self print help        sys exit -1   def  args get args sys argv 1         parser   argparser ArgumentParser        parser error    error               will output        AttributeError   str  object has no attribute  print help        You can pass parser  self  in  error function  by calling it   def  error self  message       self print help        sys exit -1   def  args get args sys argv 1         parser   argparser ArgumentParser        parser error    error parser                  but you don t want exit the program  right now  Then return it   def  error parser       def wrapper            parser print help            sys exit -1       return wrapper         Nonetheless  parser doesn t know  that it has been modified  thus when an error occurs  it will send cause of it  by the way  its localized translation   Well  then intercept it   def  error parser       def wrapper interceptor           parser print help            sys exit -1       return wrapper         Now  when error occurs and parser will send cause of it  you ll intercept it  look at this  and    throw out

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The cleanest solution will be to manually pass default argument if none were given on the command line   parser parse args args None if sys argv 1   else   --help      Complete example   import argparse  sys  parser   argparse ArgumentParser   parser add argument  --host   default  localhost   help  Host to connect to     parse arguments args   parser parse args args None if sys argv 1   else   --help       use your args print  connecting to     format args host     This will print complete help  not short usage  if called w o arguments

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parser print help   parser exit     The parser exit method also accept a status  returncode   and a message value  include a trailing newline yourself    an opinionated example        usr bin env python3   quot  quot  quot  Example argparser based python file  quot  quot  quot   import argparse  ARGP   argparse ArgumentParser      description   doc        formatter class argparse RawTextHelpFormatter    ARGP add argument  --example   action  store true   help  Example Argument     def main argp None       if argp is None          argp   ARGP parse args      pragma  no cover      if  soemthing went wrong  and not argp example          ARGP print help           ARGP exit status 64  message  quot  nSomething went wrong  --example condition was not set n quot     if   name         main         main      pragma  no cover  Example calls     python3   helloworld py  echo    usage  helloworld py  -h   --example    Example argparser based python file  optional arguments    -h  --help  show this help message and exit   --example   Example Argument  Something went wrong  --example condition was not set 64   python3   helloworld py --example  echo    0

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With argparse you could do   parser argparse ArgumentParser    parser add args here   sys argv includes a list of elements starting with the program if len sys argv   lt  2      parser print usage       sys exit 1

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Throwing my version into the pile here   import argparse  parser   argparse ArgumentParser   args   parser parse args   if not vars args       parser print help       parser exit 1    You may notice the parser exit - I mainly do it like that because it saves an import line if that was the only reason for sys in the file

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If you have arguments that must be specified for the script to run - use the required parameter for ArgumentParser as shown below -  parser add argument  --foo   required True    parse args   will report an error if the script is run without any arguments

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Instead of writing a class  a try except can be used instead  try      options   parser parse args   except      parser print help       sys exit 0    The upside is that the workflow is clearer and you don t need a stub class  The downside is that the first  usage  line is printed twice   This will need at least one mandatory argument  With no mandatory arguments  providing zero args on the commandline is valid

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If you associate default functions for  sub parsers  as is mentioned under add subparsers  you can simply add it as the default action   parser   argparse ArgumentParser   parser set defaults func lambda x  parser print usage    args   parser parse args   args func args    Add the try-except if you raise exceptions due to missing positional arguments

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I like to keep things as simple as possible  this works great     usr bin env python3 Description    quot  quot  quot Tool description quot  quot  quot  Epilog     quot  quot  quot toolname py -a aflag -b bflag  with these combined it does blah quot  quot  quot  arg parser   argparse ArgumentParser      formatter class argparse RawDescriptionHelpFormatter      description Description       epilog Epilog        try          if len sys argv     1              arg parser print help       except Exception as e          print e   This is how I start all my tools as its always good to include some examples

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Set your positional arguments with nargs  and check if positional args are empty   import argparse parser   argparse ArgumentParser   parser add argument  file   nargs      args   parser parse args   if not args file      parser print help     Reference Python nargs

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When call add subparsers method save the first positional argument to dest  and check value after argparse has been initialized  like this  subparsers   parser add subparsers dest  command   And just check this this variable  if not args command      parser print help       parser exit 1     If exit   - exit code will be zero  no error   Full example     usr bin env python   quot  quot  quot  doc  quot  quot  quot   import argparse import sys  parser   argparse ArgumentParser description   doc    subparsers   parser add subparsers dest  command                                      help  List of commands    list parser   subparsers add parser  list                                       help  List contents   list parser add argument  dir   action  store                            help  Directory to list    create parser   subparsers add parser  create                                         help  Create a directory   create parser add argument  dirname   action  store                              help  New directory to create   create parser add argument  --read-only   default False  action  store true                              help  Set permissions to prevent writing to the directory    args   parser parse args    if not args command      parser print help       parser exit 1   print vars args      For debug

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Here is another way to do it  if you need something flexible where you want to display help if specific params are passed  none at all or more than 1 conflicting arg   import argparse import sys  def main        parser   argparse ArgumentParser       parser add argument  -d    --days   required False   help  Check mapped inventory that is x days old   default None      parser add argument  -e    --event   required False  action  store   dest  event id                           help  Check mapped inventory for a specific event   default None      parser add argument  -b    --broker   required False  action  store   dest  broker id                           help  Check mapped inventory for a broker   default None      parser add argument  -k    --keyword   required False  action  store   dest  event keyword                           help  Check mapped inventory for a specific event keyword   default None      parser add argument  -p    --product   required False  action  store   dest  product id                           help  Check mapped inventory for a specific product   default None      parser add argument  -m    --metadata   required False  action  store   dest  metadata                           help  Check mapped inventory for specific metadata  good for debugging past tix   default None      parser add argument  -u    --update   required False  action  store true   dest  make updates                           help  Update the event for a product if there is a difference  default No   default False      args   parser parse args        days   args days     event id   args event id     broker id   args broker id     event keyword   args event keyword     product id   args product id     metadata   args metadata     make updates   args make updates      no change counter   0     change counter   0      req arg   bool days    bool event id    bool broker id    bool product id    bool event keyword    bool metadata      if not req arg          print  Need to specify days  broker id  event id  event keyword or past tickets full metadata           parser print help           sys exit       elif req arg    1          print  More than one option specified  Need to specify only one required option           parser print help           sys exit          Processing logic here       Cheers

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If your command is something where a user needs to choose some action  then use a mutually exclusive group with required True   This is kind of an extension to the answer given by pd321   import argparse  parser   argparse ArgumentParser   group   parser add mutually exclusive group required True  group add argument  --batch   action  store   type int   metavar  pay id   group add argument  --list   action  store true   group add argument  --all   action  store true   help  check all payments    args parser parse args    if args batch      print  batch     format args batch    if args list      print  list    if args all      print  all     Output        python3 a test py    usage  a test py  -h   --batch pay id   --list   --all     a test py  error  one of the arguments --batch --list --all is required   This only give the basic help   And some of the other answers will give you the full help    But at least your users know they can do -h

[python] Display help message with python argparse when script is called without any arguments

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