Parsing boolean values with argparse

Question

I would like to use argparse to parse boolean command-line arguments written as  --foo True  or  --foo False   For example   my program --my boolean flag False   However  the following test code does not do what I would like   import argparse parser   argparse ArgumentParser description  My parser   parser add argument  --my bool   type bool  cmd line     --my bool    False   parsed args   parser parse cmd line    Sadly  parsed args my bool evaluates to True  This is the case even when I change cmd line to be   --my bool        which is surprising  since bool     evalutates to False   How can I get argparse to parse  False    F   and their lower-case variants to be False

User · Answer

I think the most canonical way will be   parser add argument  --ensure   nargs      default None   ENSURE   config ensure is None

User · Answer

I was looking for the same issue  and imho the pretty solution is    def str2bool v     return v lower   in   yes    true    t    1     and using that to parse the string to boolean as suggested above

User · Answer

A simpler way would be to use as below   parser add argument  --feature   type lambda s  s lower   in   true    t    yes    1

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Here is another variation without extra row s to set default values  The boolean value is always assigned  so that it can be used in logical statements without checking beforehand  import argparse parser   argparse ArgumentParser description  quot Parse bool quot   parser add argument  quot --do-something quot   default False  action  quot store true quot                       help  quot Flag to do something quot   args   parser parse args    if args do something       print  quot Do something quot   else       print  quot Don t do something quot    print f quot Check that args do something  args do something  is always a bool  quot

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This works for everything I expect it to   add boolean argument parser   foo   default True  parser parse args                         Whatever the default was parser parse args   --foo                 True parser parse args   --nofoo               False parser parse args   --foo true            True parser parse args   --foo false           False parser parse args   --foo    --nofoo      Error   The code   def  str to bool s          Convert string to bool  in argparse context          if s lower   not in   true    false            raise ValueError  Need bool  got  r    s      return   true   True   false   False  s lower     def add boolean argument parser  name  default False                                                                                                         Add a boolean argument to an ArgumentParser instance         group   parser add mutually exclusive group       group add argument           --    name  nargs      default default  const True  type  str to bool      group add argument  --no    name  dest name  action  store false

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class FlagAction argparse Action         From http   bugs python org issue8538      def   init   self  option strings  dest  default None                   required False  help None  metavar None                   positive prefixes   --    negative prefixes   --no-             self positive strings   set           self negative strings   set           for string in option strings              assert re match r -- A-z     string              suffix   string 2               for positive prefix in positive prefixes                  self positive strings add positive prefix   suffix              for negative prefix in negative prefixes                  self negative strings add negative prefix   suffix          strings   list self positive strings   self negative strings          super FlagAction  self    init   option strings strings  dest dest                                           nargs 0  const None  default default  type bool  choices None                                           required required  help help  metavar metavar       def   call   self  parser  namespace  values  option string None           if option string in self positive strings              setattr namespace  self dest  True          else              setattr namespace  self dest  False

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There seems to be some confusion as to what type bool and type  bool  might mean   Should one  or both  mean  run the function bool    or  return a boolean    As it stands type  bool  means nothing   add argument gives a  bool  is not callable error  same as if you used type  foobar   or type  int    But argparse does have registry that lets you define keywords like this   It is mostly used for action  e g   action  store true    You can see the registered keywords with   parser  registries   which displays a dictionary    action    None  argparse  StoreAction     append   argparse  AppendAction     append const   argparse  AppendConstAction        type    None   lt function argparse identity gt      There are lots of actions defined  but only one type  the default one  argparse identity   This code defines a  bool  keyword   def str2bool v      susendberg s function   return v lower   in   yes    true    t    1   p   argparse ArgumentParser   p register  type   bool  str2bool    add type keyword to registries p add argument  -b  type  bool      do not use  type bool    p add argument  -b  type str2bool    works just as well p parse args  -b false  split    Namespace b False    parser register   is not documented  but also not hidden   For the most part the programmer does not need to know about it because type and action take function and class values   There are lots of stackoverflow examples of defining custom values for both     In case it isn t obvious from the previous discussion  bool   does not mean  parse a string    From the Python documentation       bool x   Convert a value to a Boolean  using the standard truth testing procedure     Contrast this with      int x   Convert a number or string x to an integer

User · Answer

In addition to what  mgilson said  it should be noted that there s also a ArgumentParser add mutually exclusive group required False  method that would make it trivial to enforce that --flag and --no-flag aren t used at the same time

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Convert the value  def   arg to bool   arg        quot  quot  quot   arg to bool            Convert string   int arg to bool      param arg  argument to be converted      type arg  str or int      return  converted arg      rtype  bool      quot  quot  quot      str true values              1            ENABLED            ON            TRUE            YES             str false values              0            DISABLED            OFF            FALSE            NO              if isinstance arg  str           arg   arg upper           if arg in str true values              return True         elif arg in str false values              return False      if isinstance arg  int           if arg    1              return True         elif arg    0              return False      if isinstance arg  bool           return arg        if any other value not covered above  consider argument as False       or you could just raise and error     return False          args   ap parse args   my arg   options my arg my arg     arg to bool   my arg

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If you want to allow --feature and --no-feature at the same time  last one wins  This allows users to make a shell alias with --feature  and overriding it with --no-feature  Python 3 9 and above parser add argument  --feature   default True  action argparse BooleanOptionalAction   Python 3 8 and below I recommend mgilson s answer  parser add argument  --feature   dest  feature   action  store true   parser add argument  --no-feature   dest  feature   action  store false   parser set defaults feature True   If you DON T want to allow --feature and --no-feature at the same time You can use a mutually exclusive group  feature parser   parser add mutually exclusive group required False  feature parser add argument  --feature   dest  feature   action  store true   feature parser add argument  --no-feature   dest  feature   action  store false   parser set defaults feature True   You can use this helper if you are going to set many of them  def add bool arg parser  name  default False       group   parser add mutually exclusive group required False      group add argument  --    name  dest name  action  store true       group add argument  --no-    name  dest name  action  store false       parser set defaults    name default    add bool arg parser   useful-feature   add bool arg parser   even-more-useful-feature

User · Answer

Yet another solution using the previous suggestions  but with the  correct  parse error from argparse   def str2bool v       if isinstance v  bool          return v     if v lower   in   yes    true    t    y    1            return True     elif v lower   in   no    false    f    n    0            return False     else          raise argparse ArgumentTypeError  Boolean value expected      This is very useful to make switches with default values  for instance  parser add argument  --nice   type str2bool  nargs                              const True  default False                          help  Activate nice mode      allows me to use   script --nice script --nice  lt bool gt    and still use a default value  specific to the user settings   One  indirectly related  downside with that approach is that the  nargs  might catch a positional argument -- see this related question and this argparse bug report

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Simplest  It s not flexible  but I prefer simplicity      parser add argument  --boolean flag                         help  This is a boolean flag                          type eval                         choices  True  False                          default  True     EDIT  If you don t trust the input  don t use eval

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I think a more canonical way to do this is via   command --feature   and  command --no-feature   argparse supports this version nicely   parser add argument  --feature   dest  feature   action  store true   parser add argument  --no-feature   dest  feature   action  store false   parser set defaults feature True      Of course  if you really want the --arg  lt True False gt  version  you could pass ast literal eval as the  type   or a user defined function      def t or f arg       ua   str arg  upper       if  TRUE  startswith ua          return True     elif  FALSE  startswith ua          return False     else         pass   error condition maybe

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oneliner   parser add argument  --is debug   default False  type lambda x   str x  lower       true

User · Answer

just do the following   you can make --test   True by using python filename --test parser add argument  quot --test quot    default False  help  quot test   quot   dest  test   action  store true

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Quick and easy  but only for arguments 0 or 1    parser add argument  mybool   default True type lambda x  bool int x    myargs parser parse args   print myargs mybool    The output will be  False  after calling from terminal   python myscript py 0

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As an improvement to  Akash Desarda  s answer  you could do   import argparse from distutils util import strtobool  parser   argparse ArgumentParser   parser add argument  --foo        type lambda x bool strtobool x        nargs      const True  default False  args   parser parse args   print args foo    And it supports python test py --foo   base   costa costa-pc code   python test py False  base   costa costa-pc code   python test py --foo  True  base   costa costa-pc code   python test py --foo True True  base   costa costa-pc code   python test py --foo False False

User · Answer

Simplest way would be to use choices   parser   argparse ArgumentParser   parser add argument  --my-flag  choices   True   False     args   parser parse args   flag   args my flag     True  print flag    Not passing --my-flag evaluates to False   The required True option could be added if you always want the user to explicitly specify a choice

User · Answer

A quite similar way is to use   feature add argument  --feature  action  store true     and if you set the argument --feature in your command    command --feature   the argument will be True  if you do not set type --feature the arguments default is always False

User · Answer

Simplest  amp  most correct way is  from distutils util import strtobool  parser add argument  --feature   dest  feature                        type lambda x  bool strtobool x     Do note that True values are y  yes  t  true  on and 1  false values are n  no  f  false  off and 0  Raises ValueError if val is anything else

User · Answer

Similar to  Akash but here is another approach that I ve used  It uses str than lambda because python lambda always gives me an alien-feelings   import argparse from distutils util import strtobool  parser   argparse ArgumentParser   parser add argument  --my bool   type str  default  False   args   parser parse args    if bool strtobool args my bool   is True      print  OK

[python] Parsing boolean values with argparse

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