Use and meaning of in in an if statement

Question

In an example from Zed Shaw s Learn Python the Hard Way  one of the exercises displays the following code   next   raw input   gt     if  0  in next or  1  in next      how much   int next    I m having a hard time understanding the meaning of in in this statement  I m used to using if statements  such as in javascript  where the syntax is something like   var   5  if  var  gt  3          code  to be executed     Is this if   in statement  in python  the same as if   in javascript   Finding an answer to this has been tricky because the in is such a short string to narrow down an answer via search engine  and I don t know the proper name for its operation

User · Answer

This may be a very late answer   in operator checks for memberships  That is  it checks if its left operand is a member of its right operand   In this case  raw input   returns an str object of what is supplied by the user at the standard input   So  the if condition checks whether the input contains substrings  0  or  1   Considering the typecasting  int    in the following line  the if condition essentially checks if the input contains digits 0 or 1

User · Answer

It depends on what next is   If it s a string  as in your example   then in checks for substrings    gt  gt  gt   in  in  indigo  True  gt  gt  gt   in  in  violet  False  gt  gt  gt   0  in  10  True  gt  gt  gt   1  in  10  True   If it s a different kind of iterable  list  tuple  set  dictionary      then in checks for membership    gt  gt  gt   in  in   in    out   True  gt  gt  gt   in  in   indigo    violet   False   In a dictionary  membership is seen as  being one of the keys     gt  gt  gt   in  in   in    out   True  gt  gt  gt   in  in   out    in   False

User · Answer

Here raw input is string  so if you wanted to check  if var gt 3 then you should convert next to double  ie float next  and do as you would do if float next  gt 3   but in most cases

User · Answer

You are used to using the javascript if  and I assume you know how it works   in is a Pythonic way of implementing iteration  It s supposed to be easier for non-programmatic thinkers to adopt  but that can sometimes make it harder for programmatic thinkers  ironically   When you say if x in y  you are literally saying    if x is in y   which assumes that y has an index  In that if statement then  each object at each index in y is checked against the condition   Similarly   for x in y  iterates through x s in y  where y is that indexable set of items   Think of the if situation this way  pseudo-code    for i in next      if i     0     i     1           how much   int next    It takes care of the iteration over next for you   Happy coding

User · Answer

the reserved word  in  is used to look inside an object that can be iterated over   list obj     a    b    c   tuple obj     a   1  2 0  dict obj     a   1   b   2 0  obj to find    c  if obj to find in list obj      print  Object  0  is in  1   format obj to find  list obj   obj to find   2 0 if obj to find in tuple obj      print  Object  0  is in  1   format obj to find  tuple obj   obj to find    b  if obj to find in dict obj      print  Object  0  is in  1   format obj to find  dict obj     Output   Object c is in   a    b    c   Object 2 0 is in   a   1  2 0  Object b is in   a   1   b   2 0    However   cannot iterate over   5 5 obj to find   5 5 if obj to find in cannot iterate over      print  Object  0  is in  1   format obj to find  cannot iterate over     will throw  Traceback  most recent call last   File   home jgranger workspace sandbox src csv file creator py   line 43  in  lt module gt  if obj to find in cannot iterate over  TypeError  argument of type  float  is not iterable   In your case  raw input       returns iterable object or it will throw TypeError

User · Answer

Maybe these examples will help illustrate what in does  It basically translate to Is this item in this other item   listOfNums     1  2  3  4  5  6  45   j      gt  gt  gt  3 in listOfNums   gt  gt  gt  True   gt  gt  gt   j  in listOfNums   gt  gt  gt  True   gt  gt  gt  66 in listOfNums   gt  gt  gt  False

User · Answer

Using a in b is simply translates to b   contains   a   which should return if b includes a or not   But  your example looks a little weird  it takes an input from user and assigns its integer value to how much variable if the input contains  0  or  1

User · Answer

Since you claim to be used to JavaScript   The Python in operator is similar to the JavaScript in operator   Here s some JavaScript   var d    1  2  3  4   if  1 in d        alert  true         And the equivalent Python   d    1  2  3  4  if 1 in d      print  true      With objects dicts  they re nearly identical  both checking whether 1 is a key of the object dict  The big difference  of course  is that JavaScript is sloppily-typed  so  1  in d would be just as true   With arrays lists  they re very different  A JS array is an object  and its indexes are the keys  so 1 in  3  4  5  will be true  A Python list is completely different from a dict  and its in operator checks the values  not the indexes  which tends to be more useful  And Python extends this behavior to all iterables   With strings  they re even more different  A JS string isn t an object  so you will get a TypeError  But a Python str or unicode will check whether the other operand is a substring   This means 1 in  123  is illegal  because 1 can t be a substring of anything  but  1  in  123  is true    With objects as objects  in JS there is of course no distinction  but in Python  objects are instances of classes  not dicts  So  in JS  1 in d will be true for an object if it has a member or method named  1   but in Python  it s up to your class what it means   Python will call d   contains   1   then  if that fails  it tries to use your object as an utterable  by calling its   iter    and  if that fails  by trying to index it with integers starting from 0    Also  note that JS s in  because it s actually checking for object membership  does the usual JS method-resolution-order search  while Python s in  because it s checking for keys of a dict  members of a sequence  etc   does no such thing  So  technically  it s probably a bit closer to the hasOwnProperty method than the in operator

[python] Use and meaning of "in" in an if statement?

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