Is there a Python equivalent of the C null-coalescing operator

Question

In C  there s a null-coalescing operator  written as     that allows for easy  short  null checking during assignment   string s   null  var other   s     some default value     Is there a python equivalent   I know that I can do   s   None other   s if s else  some default value    But is there an even shorter way  where I don t need to repeat s

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In addition to Juliano s answer about behavior of  or    it s  fast    gt  gt  gt  1 or 5 0 1   So sometimes it s might be a useful shortcut for things like  object   getCachedVersion   or getFromDB

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I realize this is answered  but there is another option when you re dealing with objects    If you have an object that might be         name          first   John         last   Doe           You can use    obj get property name  value if null    Like    obj get  name       get  first    Name is missing      By adding    as the default value  if  name  is missing  an empty object is returned and passed through to the next get  This is similar to null-safe-navigation in C   which would be like obj  name  first

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Strictly   other   s if s is not None else  default value    Otherwise  s   False will become  default value   which may not be what was intended   If you want to make this shorter  try   def notNone s d       if s is None          return d     else          return s  other   notNone s   default value

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The two functions below I have found to be very useful when dealing with many variable testing cases    def nz value  none value  strict True           This function is named after an old VBA function  It returns a default         value if the passed in value is None  If strict is False it will         treat an empty string as None as well           example          x   None         nz x  hello           -- gt   hello          nz x             -- gt             y                 nz y  hello           -- gt             nz y  hello   False          -- gt   hello           if value is None and strict          return val   none value     elif strict and value is not None          return val   value     elif not strict and not is not null value           return val   none value     else          return val   value     return return val   def is not null value           test for None and empty string         return value is not None and len str value    gt  0

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In case you need to nest more than one null coalescing operation such as   model  data    first    This is not a problem easily solved with or  It also cannot be solved with  get   which requires a dictionary type or similar  and cannot be nested anyway  or getattr   which will throw an exception when NoneType doesn t have the attribute    The relevant pip considering adding null coalescing to the language is PEP 505 and the discussion relevant to the document is in the python-ideas thread

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Regarding answers by  Hugh Bothwell   mortehu and  glglgl   Setup Dataset for testing  import random  dataset    random randint 0 15  if random random    gt   6 else None for i in range 1000     Define implementations  def not none x  y None       if x is None          return y     return x  def coalesce1  arg     return reduce lambda x  y  x if x is not None else y  arg   def coalesce2  args       return next  i for i in args if i is not None   None    Make test function  def test func dataset  func       default   1     for i in dataset          func i  default    Results on mac i7  2 7Ghz using python 2 7   gt  gt  gt   timeit test func dataset  not none  1000 loops  best of 3  224   s per loop   gt  gt  gt   timeit test func dataset  coalesce1  1000 loops  best of 3  471   s per loop   gt  gt  gt   timeit test func dataset  coalesce2  1000 loops  best of 3  782   s per loop   Clearly the not none function answers the OP s question correctly and handles the  falsy  problem  It is also the fastest and easiest to read  If applying the logic in many places  it is clearly the best way to go   If you have a problem where you want to find the 1st non-null value in a iterable  then  mortehu s response is the way to go  But it is a solution to a different problem than OP  although it can partially handle that case  It cannot take an iterable AND a default value  The last argument would be the default value returned  but then you wouldn t be passing in an iterable in that case as well as it isn t explicit that the last argument is a default to value    You could then do below  but I d still use not null for the single value use case   def coalesce  args    kwargs       default   kwargs get  default       return next  a for a in arg if a is not None   default

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other   s or  some default value    Ok  it must be clarified how the or operator works  It is a boolean operator  so it works in a boolean context  If the values are not boolean  they are converted to boolean for the purposes of the operator   Note that the or operator does not return only True or False  Instead  it returns the first operand if the first operand evaluates to true  and it returns the second operand if the first operand evaluates to false   In this case  the expression x or y returns x if it is True or evaluates to true when converted to boolean  Otherwise  it returns y  For most cases  this will serve for the very same purpose of C  s null-coalescing operator  but keep in mind   42    or  something       returns 42 0     or  something       returns  something  None  or  something       returns  something  False or  something       returns  something        or  something       returns  something    If you use your variable s to hold something that is either a reference to the instance of a class or None  as long as your class does not define members   nonzero     and   len       it is secure to use the same semantics as the null-coalescing operator   In fact  it may even be useful to have this side-effect of Python  Since you know what values evaluates to false  you can use this to trigger the default value without using None specifically  an error object  for example    In some languages this behavior is referred to as the Elvis operator

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For those like me that stumbled here looking for a viable solution to this issue  when the variable might be undefined  the closest i got is   if  variablename  in globals   and   variablename or False     True     print  variable exists and it  s true   else    print  variable doesn  t exist  or it  s false     Note that a string is needed when checking in globals  but afterwards the actual variable is used when checking for value   More on variable existence  How do I check if a variable exists

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Python has a get function that its very useful to return a value of an existent key  if the key exist  if not it will return a default value   def main        names     Jack   Maria   Betsy   James   Jack       names repeated   dict       default value   0      for find name in names          names repeated find name    names repeated get find name  default value    1   if you cannot find the name inside the dictionary  it will return the default value   if the name exist then it will add any existing value with 1   hope this can help

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Addionally to  Bothwells answer  which I prefer  for single values  in order to null-checking assingment of function return values  you can use new walrus-operator  since python3 8    def test        return  a   2 if  x   test    is None else x   Thus  test function does not need to be evaluated two times  as in a   2 if test   is None else test

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Here s a function that will return the first argument that isn t None   def coalesce  arg     return reduce lambda x  y  x if x is not None else y  arg     Prints  banana  print coalesce None   banana    phone   None    reduce   might needlessly iterate over all the arguments even if the first argument is not None  so you can also use this version   def coalesce  arg     for el in arg      if el is not None        return el   return None

[python] Is there a Python equivalent of the C# null-coalescing operator?

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