Checking if sys argv x is defined

Question

What would be the best way to check if a variable was passed along for the script   try      sys argv 1  except NameError      startingpoint    blah  else      startingpoint   sys argv 1

User · Answer

Check the length of sys argv   if len sys argv   gt  1      blah   sys argv 1  else      blah    blah    Some people prefer the exception-based approach you ve suggested  eg  try  blah   sys argv 1   except IndexError  blah    blah    but I don t like it as much because it doesn t    scale    nearly as nicely  eg  when you want to accept two or three arguments  and it can potentially hide errors  eg  if you used blah   foo sys argv 1    but foo      raised an IndexError  that IndexError would be ignored

User · Answer

Another way I haven t seen listed yet is to set your sentinel value ahead of time  This method takes advantage of Python s lazy evaluation  in which you don t always have to provide an else statement   Example   startingpoint    blah  if len sys argv   gt   2    startingpoint   sys argv 1    Or if you re going syntax CRAZY you could use Python s ternary operator   startingpoint   sys argv 1  if len sys argv   gt   2 else  blah

User · Answer

You can simply append the value of argv 1  to argv  and then check if argv 1  doesn t equal the string you inputted Example   from sys import argv argv append  SomeString   if argv 1    SomeString               print argv 1

User · Answer

Pretty close to what the originator was trying to do  Here is a function I use   def get arg index       try          sys argv index      except IndexError          return        else          return sys argv index    So a usage would be something like   if   name         main         banner get arg 1  get arg 2

User · Answer

In the end  the difference between try  except and testing len sys argv  isn t all that significant  They re both a bit hackish compared to argparse   This occurs to me  though -- as a sort of low-budget argparse   arg names     command    x    y    operation    option   args   dict zip arg names  sys argv     You could even use it to generate a namedtuple with values that default to None -- all in four lines   Arg list   collections namedtuple  Arg list   arg names  args   Arg list   args get arg  None  for arg in arg names     In case you re not familiar with namedtuple  it s just a tuple that acts like an object  allowing you to access its values using tup attribute syntax instead of tup 0  syntax    So the first line creates a new namedtuple type with values for each of the values in arg names  The second line passes the values from the args dictionary  using get to return a default value when the given argument name doesn t have an associated value in the dictionary

User · Answer

I use this - it never fails   startingpoint    blah  if sys argv 1       startingpoint   sys argv 1

User · Answer

It s an ordinary Python list  The exception that you would catch for this is IndexError  but you re better off just checking the length instead   if len sys argv   gt   2    startingpoint   sys argv 1  else    startingpoint    blah

User · Answer

A solution working with map built-in fonction    arg names     command    operation    parameter   args   map None  arg names  sys argv  args    k v for  k v  in args    Then you just have to call your parameters like this   if args  operation       division       if not args  parameter                    if args  parameter       euclidian

[python] Checking if sys.argv[x] is defined

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