Python Assign Value if None Exists

Question

I am a RoR programmer new to Python  I am trying to find the syntax that will allow me to set a variable to a specific value only if it wasn t previously assigned  Basically I want     only if var1 has not been previously assigned  var1   4

User · Answer

One-liner solution here  var1   locals   get  quot var1 quot    quot default value quot    Instead of having NameError  this solution will set var1 to default value if var1 hasn t been defined yet  Here s how it looks like in Python interactive shell   gt  gt  gt  var1 Traceback  most recent call last     File  quot  lt stdin gt  quot   line 1  in  lt module gt  NameError  name  var1  is not defined  gt  gt  gt  var1   locals   get  quot var1 quot    quot default value 1 quot    gt  gt  gt  var1  default value 1   gt  gt  gt  var1   locals   get  quot var1 quot    quot default value 2 quot    gt  gt  gt  var1  default value 1   gt  gt  gt

User · Answer

IfLoop s answer  and MatToufoutu s comment  work great for standalone variables  but I wanted to provide an answer for anyone trying to do something similar for individual entries in lists  tuples  or dictionaries   Dictionaries  existing dict     spam   1   eggs   2  existing dict  foo     existing dict  foo   if  foo  in existing dict else 3   Returns   spam   1   eggs   2   foo   3   Lists  existing list     spam   eggs   existing list   existing list if len existing list   3 else                  existing list     foo     Returns   spam    eggs    foo    Tuples  existing tuple     spam   eggs   existing tuple   existing tuple if len existing tuple   3 else                   existing tuple     foo      Returns   spam    eggs    foo     Don t forget the comma in   foo    to define a  single  tuple    The lists and tuples solution will be more complicated if you want to do more than just check for length and append to the end  Nonetheless  this gives a flavor of what you can do

User · Answer

If you mean a variable at the module level then you can use  globals    if  var1  not in globals        var1   4   but the common Python idiom is to initialize it to say None  assuming that it s not an acceptable value  and then testing with if var1 is not None

User · Answer

You should initialize variables to None and then check it   var1   None if var1 is None      var1   4   Which can be written in one line as   var1   4 if var1 is None else var1   or using shortcut  but checking against None is recommended   var1   var1 or 4   alternatively if you will not have anything assigned to variable that variable name doesn t exist and hence using that later will raise NameError  and you can also use that knowledge to do something like this  try      var1 except NameError      var1   4   but I would advise against that

User · Answer

This is a very different style of programming  but I always try to rewrite things that looked like  bar   None if foo        bar    Baz   if bar is None      bar    Quux    into just   if foo        bar    Baz  else      bar    Quux    That is to say  I try hard to avoid a situation where some code paths define variables but others don t   In my code  there is never a path which causes an ambiguity of the set of defined variables  In fact  I usually take it a step further and make sure that the types are the same regardless of code path    It may just be a matter of personal taste  but I find this pattern  though a little less obvious when I m writing it  much easier to understand when I m later reading it

User · Answer

Here is the easiest way I use  hope works for you   var1   var1 or 4  This assigns 4 to var1 only if var1 is None   False or 0

User · Answer

var1   var1 or 4   The only issue this might have is that if var1 is a falsey value  like False or 0 or     it will choose 4 instead  That might be an issue

User · Answer

I m also coming from Ruby so I love the syntax foo     7   This is the closest thing I can find   foo   foo if  foo  in vars   else 7   I ve seen people do this for a dict   try      foo  bar   except KeyError      foo  bar     7   Upadate  However  I recently found this gem   foo  bar     foo get  bar   7    If you like that  then for a regular variable you could do something like this   vars    foo     vars   get  foo   7

[python] Python: Assign Value if None Exists

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