Python conditional assignment operator

Question

Does a Python equivalent to the Ruby     operator   set the variable if the variable is not set   exist   Example in Ruby     variable not set      bla bla   variable not set     bla bla    variable set    pi pi   variable set      bla bla   variable set     pi pi

User · Answer

I am not sure I understand the question properly here ... Trying to "read" the value of an "undefined" variable name will trigger a NameError. (see here, that Python has "names", not variables...).

== EDIT ==

As pointed out in the comments by delnan, the code below is not robust and will break in numerous situations ...

Nevertheless, if your variable "exists", but has some sort of dummy value, like None, the following would work :

>>> my_possibly_None_value = None
>>> myval = my_possibly_None_value or 5
>>> myval
5
>>> my_possibly_None_value = 12
>>> myval = my_possibly_None_value or 5
>>> myval
12
>>>

(see this paragraph about Truth Values)

User · Answer

No  the replacement is   try     v except NameError     v    bla bla    However  wanting to use this construct is a sign of overly complicated code flow  Usually  you d do the following   try     v   complicated   except ComplicatedError    complicated failed    v    fallback value    and never be unsure whether v is set or not  If it s one of many options that can either be set or not  use a dictionary and its get method which allows a default value

User · Answer

No  not knowing which variables are defined is a bug  not a feature in Python    Use dicts instead   d      d setdefault  key   1  d  key      1  d  key     2 d setdefault  key   1  d  key      2

User · Answer

No  there is no nonsense like that   Something we have not missed in Python for 20 years

User · Answer

I would use   x    default  if not x else x   Much shorter than all of your alternatives suggested here  and straight to the point  Read   set x to  default  if x is not set otherwise keep it as x   If you need None  0  False  or    to be valid values however  you will need to change this behavior  for instance    valid vals        0  False    We want None to be the only un-set value  x    default  if not x and x not in valid vals else x   This sort of thing is also just begging to be turned into a function you can use everywhere easily    setval if   lambda val   default  if not val and val not in valid vals else val   at which point  you can use it as    gt  gt  gt  x   None   To set it to something not valid  gt  gt  gt  x   setval if x    Using our special function is short and sweet now   gt  gt  gt  print x   Let s check to make sure our None valued variable actually got set  default    Finally  if you are really missing your Ruby infix notation  you could overload       or something similar  by following this guy s hack  http   code activestate com recipes 384122-infix-operators

User · Answer

I m surprised no one offered this answer  It s not as  built-in  as Ruby s     but it s basically equivalent and still a one-liner   foo   foo if  foo  in locals   else  default    Of course  locals   is just a dictionary  so you can do   foo   locals   get  foo    default

User · Answer

can t comment or I would just do that  I believe the suggestion to check locals above is not quite right   It should be   foo   foo if  foo  in locals   or  foo  in globals   else  default    to be correct in all contexts     However  despite its upvotes  I don t think even that is a good analog to the Ruby operator   Since the Ruby operator allows more than just a simple name on the left   foo 12      something foo bar     something   The exception method is probably closest analog

User · Answer

There is conditional assignment in Python 2 5 and later - the syntax is not very obvious hence it s easy to miss  Here s how you do it   x   true value if condition else false value   For further reference  check out the Python 2 5 docs

User · Answer

I think what you are looking for  if you are looking for something in a dictionary  is the setdefault method    Pdb  we dict    Pdb  we setdefault  e  14  14  Pdb  we  e   14  Pdb  we  r    p   Pdb  we setdefault  r   jeff    p   Pdb  we  r    p   Pdb  we e      NameError  name  e  is not defined  Pdb  we  e   14  Pdb  we  q2         KeyError   q2       Pdb   The important thing to note in my example is that the setdefault method changes the dictionary if and only if the key that the setdefault method refers to is not present

User · Answer

I usually do this the following way   def set if not exists obj attr value    if not hasattr obj attr   setattr obj attr value

[python] Python conditional assignment operator

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