Initialize a string variable in Python or None

Question

Suppose I have a class with a string instance attribute  Should I initialize this attribute with    value or None  Is either okay   def   init   self  mystr        self mystr   mystr   or  def   init   self  mystr None     self mystr   mystr   Edit  What I thought is that if I use    as an initial value  I  declare  a variable to be of string type  And then I won t be able to assign any other type to it later  Am I right   Edit  I think it s important to note here  that my suggestion was WRONG  And there is no problem to assign another type to a variable  I liked a comment of S Lott   Since nothing in Python is  declared   you re not thinking about this the right way

User · Answer

Python philosophy is to be readable  That s why it s good practice to define your attributes in   init     even if it s optional  In the same spirit  you have to ask yourself what clearer for anyone who reads your code  In fact the type itself give much information about future use of your variable  So   kind   None   Is syntaxically correct  but reader does not know much  Is it a string  a code as integer  a list  etc     kind str   None kind        Both say a little more  the first has the type in its name and the second in its declaration  I would go for the second  neater

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None is used to indicate  not set   whereas any other value is used to indicate a  default  value   Hence  if your class copes with empty strings and you like it as a default value  use     If your class needs to check if the variable was set at all  use None   Notice that it doesn t matter if your variable is a string initially  You can change it to any other type value at any other moment

User · Answer

It depends  If you want to distinguish between no parameter passed in at all  and an empty string passed in  you could use None

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If not having a value has a meaning in your program  e g  an optional value   you should use None  That s its purpose anyway    If the value must be provided by the caller of   init    I would recommend not to initialize it   If    makes sense as a default value  use it   In Python the type is deduced from the usage  Hence  you can change the type by just assigning a value of another type    gt  gt  gt  x   None  gt  gt  gt  print type x   lt type  NoneType  gt   gt  gt  gt  x    text   gt  gt  gt  print type x   lt type  str  gt   gt  gt  gt  x   42  gt  gt  gt  print type x   lt type  int  gt

User · Answer

Either is fine  though None is more common as a convention - None indicates that no value was passed for the optional parameter   There will be times when    is the correct default value to use - in my experience  those times occur less often

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Since both None and    are false  you can do both  See 6 1  Truth Value Testing   Edit  To answer the question in your edit  No  you can assign a different type    gt  gt  gt  a       gt  gt  gt  type a   lt type  str  gt   gt  gt  gt  a   1  gt  gt  gt  type a   lt type  int  gt

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Either might be fine  but I don t think there is a definite answer    If you want to indicate that the value has not been set  comparing with None is better than comparing with     since    might be a valid value  If you just want a default value     is probably better  because its actually a string  and you can call string methods on it  If you went with None  these would lead to exceptions  If you wish to indicate to future maintainers that a string is required here     can help with that    Complete side note   If you have a loop  say   def myfunc  self  mystr            for other in self strs          mystr   self otherfunc  mystr  other    then a potential future optimizer would know that str is always a string  If you used None  then it might not be a string until the first iteration  which would require loop unrolling to get the same effects  While this isn t a hypothetical  it comes up a lot in my PHP compiler  you should certainly never write your code to take this into account  I just thought it might be interesting

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For lists or dicts  the answer is more clear  according to http   python net  goodger projects pycon 2007 idiomatic handout html default-parameter-values use None as default parameter   But also for strings  a  empty  string object is instanciated at runtime for the keyword parameter   The cleanest way is probably   def myfunc self  my string None       self my string   my string or      or a if-else-branch

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Either way is okay in python   I would personally prefer      but again  either way is okay    gt  gt  gt x   None  gt  gt  gt print x  None  gt  gt  gt type x   lt class  NoneType  gt   gt  gt  gt x    hello there   gt  gt  gt print x  hello there  gt  gt  gt type x   lt class  str  gt   gt  gt  gt    gt  gt  gt x       gt  gt  gt print x    gt  gt  gt type x   lt class  str  gt   gt  gt  gt x    hello there   gt  gt  gt type x   lt class  str  gt   gt  gt  gt print x  hello there

User · Answer

empty string       if not empty string       print  Empty string is not set      Empty string is not set  if empty string is not None      print  Empty string is not None      Empty string is not None

User · Answer

Another way to initialize an empty string is by using the built-in str   function with no arguments      str object          Return a string containing a nicely printable representation of an object                If no argument is given  returns the empty string        In the original example  that would look like this   def   init   self  mystr str       self mystr   mystr   Personally  I believe that this better conveys your intentions   Notice by the way that str   itself sets a default parameter value of

[python] Initialize a string variable in Python: "" or None?

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