Check if argparse optional argument is set or not

Question

I would like to check whether an optional argparse argument has been set by the user or not   Can I safely check using isset   Something like this   if isset args myArg         do something else       do something else   Does this work the same for float   int   string type arguments   I could set a default parameter and check it  e g   set myArg   -1  or    for a string  or  NOT SET     However  the value I ultimately want to use is only calculated later in the script   So I would be setting it to -1 as a default  and then updating it to something else later  This seems a little clumsy in comparison with simply checking if the value was set by the user

User · Accepted Answer

I think that optional arguments  specified with --  are initialized to None if they are not supplied  So you can test with is not None  Try the example below   import argparse as ap  def main        parser   ap ArgumentParser description  My Script       parser add argument  --myArg       args  leftovers   parser parse known args        if args myArg is not None          print  myArg has been set  value is  s     args myArg

User · Answer

A custom action can handle this problem  And I found that it is not so complicated  is set   set    global set reference class IsStored argparse Action       def   call   self  parser  namespace  values  option string None           is set add self dest    save to global reference         setattr namespace  self dest     set   True    or you may inject directly to namespace         setattr namespace  self dest  values    implementation of store action           You cannot inject directly to self dest until you have a custom class   parser add argument  quot --myarg quot   type int  default 1  action IsStored  params   parser parse args   print params myarg   myarg  in is set  print hasattr params   myarg set

User · Answer

As  Honza notes is None is a good test   It s the default default  and the user can t give you a string that duplicates it   You can specify another default  mydefaultvalue  and test for that   But what if the user specifies that string   Does that count as setting or not   You can also specify default argparse SUPPRESS   Then if the user does not use the argument  it will not appear in the args namespace   But testing that might be more complicated   args foo   raises an AttributeError hasattr args   foo      returns False getattr args   foo    other     returns  other    Internally the parser keeps a list of seen actions  and uses it for  required  and  mutually exclusive  testing   But it isn t available to you out side of parse args

User · Answer

In order to address  kcpr s comment on the  currently accepted  answer by  Honza Osobne  Unfortunately it doesn t work then the argument got it s default value defined   one can first check if the argument was provided by comparing it with the Namespace object and providing the default argparse SUPPRESS option  see  hpaulj s and  Erasmus Cedernaes answers and this python3 doc   and if it hasn t been provided  then set it to a default value  import argparse  parser   argparse ArgumentParser   parser add argument  --infile   default argparse SUPPRESS  args   parser parse args   if  infile  in args         the argument is in the namespace  it s been provided by the user       set it to what has been provided     theinfile   args infile     print  argument   --infile   was given  set to     format theinfile   else        the argument isn t in the namespace       set it to a default value     theinfile    your default txt      print  argument   --infile   was not given  set to default     format theinfile    Usage   python3 testargparse so py argument  --infile  was not given  set to default your default txt    python3 testargparse so py --infile user file txt argument  --infile  was given  set to user file txt

User · Answer

Very simple  after defining args variable by  args   parser parse args    it contains all data of args subset variables too  To check if a variable is set or no assuming the  action  quot store true quot  is used    if args argument name       do something else       do something else

User · Answer

I think using the option default argparse SUPPRESS makes most sense  Then  instead of checking if the argument is not None  one checks if the argument is in the resulting namespace   Example     import argparse  parser   argparse ArgumentParser   parser add argument  --foo   default argparse SUPPRESS  ns   parser parse args    print  Parsed arguments      format ns   print  foo in namespace       format  foo  in ns     Usage     python argparse test py --foo 1 Parsed arguments  Namespace foo  1   foo in namespace   True   Argument is not supplied     python argparse test py Parsed arguments  Namespace   foo in namespace   False

User · Answer

If your argument is positional  ie it doesn t have a  -  or a  --  prefix  just the argument  typically a file name  then you can use the nargs parameter to do this   parser   argparse ArgumentParser description  Foo is a program that does things   parser add argument  filename   nargs      args   parser parse args    if args filename is not None      print  The file name is     format args filename   else      print  Oh well   No args  no problems

User · Answer

You can check an optionally passed flag with store true and store false argument action options   import argparse  argparser   argparse ArgumentParser   argparser add argument  -flag   dest  flag exists   action  store true    print argparser parse args       Namespace flag exists False  print argparser parse args   -flag      Namespace flag exists True    This way  you don t have to worry about checking by conditional is not None  You simply check for True or False  Read more about these options in the docs here

User · Answer

Here is my solution to see if I am using an argparse variable   import argparse  ap   argparse ArgumentParser   ap add argument  -1    --first   required True  ap add argument  -2    --second   required True  ap add argument  -3    --third   required False     Combine all arguments into a list called args args   vars ap parse args    if args  third   is not None    do something   This might give more insight to the above answer which I used and adapted to work for my program

[python] Check if argparse optional argument is set or not

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