String comparison in Python is vs

Question

I noticed a Python script I was writing was acting squirrelly  and traced it to an infinite loop  where the loop condition was while line is not     Running through it in the debugger  it turned out that line was in fact     When I changed it to      rather than is not     it worked fine    Also  is it generally considered better to just use      by default  even when comparing int or Boolean values  I ve always liked to use  is  because I find it more aesthetically pleasing and pythonic  which is how I fell into this trap      but I wonder if it s intended to just be reserved for when you care about finding two objects with the same id

User · Answer

The logic is not flawed   The statement     if x is y then x  y is also True    should never be read to mean      if x  y then x is y    It is a logical error on the part of the reader to assume that the converse of a logic statement is true   See http   en wikipedia org wiki Converse  logic

User · Answer

For all built-in Python objects  like   strings  lists  dicts  functions    etc    if x is y  then x  y is also   True    Not always   NaN is a counterexample   But usually  identity  is  implies equality        The converse is not true  Two distinct objects can have the same value      Also  is it generally considered better to just use      by default  even   when comparing int or Boolean values    You use    when comparing values and is when comparing identities   When comparing ints  or immutable types in general   you pretty much always want the former   There s an optimization that allows small integers to be compared with is  but don t rely on it   For boolean values  you shouldn t be doing comparisons at all    Instead of   if x    True        do something   write   if x        do something   For comparing against None  is None is preferred over    None      I ve always liked to use  is  because   I find it more aesthetically pleasing   and pythonic  which is how I fell into   this trap      but I wonder if it s   intended to just be reserved for when   you care about finding two objects   with the same id    Yes  that s exactly what it s for

User · Answer

See This question  Your logic in reading      For all built-in Python objects  like   strings  lists  dicts  functions    etc    if x is y  then x  y is also   True    is slightly flawed   If is applies then    will be True  but it does NOT apply in reverse     may yield True while is yields False

User · Answer

I would like to show a little example on how is and    are involved in immutable types  Try that   a   19998989890 b   19998989889  1  gt  gt  gt  a is b False  gt  gt  gt  a    b True   is compares two objects in memory     compares their values  For example  you can see that small integers are cached by Python   c   1 b   1  gt  gt  gt  b is c True   You should use    when comparing values and is when comparing identities   Also  from an English point of view   equals  is different from  is

[python] String comparison in Python: is vs. ==

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