How does Python know the values already stored in its memory?





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I want to know how Python knows (if it knows) if a value-type object is already stored in its memory (and also knows where it is).



For this code, when assigning the value 1 for b, how does it know that the value 1 is already in its memory and stores its reference in b?



>>> a = 1
>>> b = 1
>>> a is b
True









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  • Use print(hex(id(b))) to check memory address for b

    – Yusufsn
    21 mins ago













  • >>> hex(id(b))'0x7ffe705ee350' >>> hex(id(a)) '0x7ffe705ee350'

    – Just A Lone
    19 mins ago











  • the values are the same

    – Just A Lone
    18 mins ago






  • 1





    If two variables refer to the same value between -5 and 256 (as opposed to use) then by definition there is only one object.

    – Yusufsn
    18 mins ago








  • 1





    @Yusufsn No. For bigger integers (>256) it's not true.

    – ajnLJA-0184
    13 mins ago


















7















I want to know how Python knows (if it knows) if a value-type object is already stored in its memory (and also knows where it is).



For this code, when assigning the value 1 for b, how does it know that the value 1 is already in its memory and stores its reference in b?



>>> a = 1
>>> b = 1
>>> a is b
True









share|improve this question









New contributor




Just A Lone is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.





















  • Use print(hex(id(b))) to check memory address for b

    – Yusufsn
    21 mins ago













  • >>> hex(id(b))'0x7ffe705ee350' >>> hex(id(a)) '0x7ffe705ee350'

    – Just A Lone
    19 mins ago











  • the values are the same

    – Just A Lone
    18 mins ago






  • 1





    If two variables refer to the same value between -5 and 256 (as opposed to use) then by definition there is only one object.

    – Yusufsn
    18 mins ago








  • 1





    @Yusufsn No. For bigger integers (>256) it's not true.

    – ajnLJA-0184
    13 mins ago














7












7








7


1






I want to know how Python knows (if it knows) if a value-type object is already stored in its memory (and also knows where it is).



For this code, when assigning the value 1 for b, how does it know that the value 1 is already in its memory and stores its reference in b?



>>> a = 1
>>> b = 1
>>> a is b
True









share|improve this question









New contributor




Just A Lone is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.












I want to know how Python knows (if it knows) if a value-type object is already stored in its memory (and also knows where it is).



For this code, when assigning the value 1 for b, how does it know that the value 1 is already in its memory and stores its reference in b?



>>> a = 1
>>> b = 1
>>> a is b
True






python python-3.x memory






share|improve this question









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Just A Lone is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











share|improve this question









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share|improve this question








edited 8 mins ago









user11206537

149115




149115






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asked 25 mins ago









Just A LoneJust A Lone

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384




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  • Use print(hex(id(b))) to check memory address for b

    – Yusufsn
    21 mins ago













  • >>> hex(id(b))'0x7ffe705ee350' >>> hex(id(a)) '0x7ffe705ee350'

    – Just A Lone
    19 mins ago











  • the values are the same

    – Just A Lone
    18 mins ago






  • 1





    If two variables refer to the same value between -5 and 256 (as opposed to use) then by definition there is only one object.

    – Yusufsn
    18 mins ago








  • 1





    @Yusufsn No. For bigger integers (>256) it's not true.

    – ajnLJA-0184
    13 mins ago



















  • Use print(hex(id(b))) to check memory address for b

    – Yusufsn
    21 mins ago













  • >>> hex(id(b))'0x7ffe705ee350' >>> hex(id(a)) '0x7ffe705ee350'

    – Just A Lone
    19 mins ago











  • the values are the same

    – Just A Lone
    18 mins ago






  • 1





    If two variables refer to the same value between -5 and 256 (as opposed to use) then by definition there is only one object.

    – Yusufsn
    18 mins ago








  • 1





    @Yusufsn No. For bigger integers (>256) it's not true.

    – ajnLJA-0184
    13 mins ago

















Use print(hex(id(b))) to check memory address for b

– Yusufsn
21 mins ago







Use print(hex(id(b))) to check memory address for b

– Yusufsn
21 mins ago















>>> hex(id(b))'0x7ffe705ee350' >>> hex(id(a)) '0x7ffe705ee350'

– Just A Lone
19 mins ago





>>> hex(id(b))'0x7ffe705ee350' >>> hex(id(a)) '0x7ffe705ee350'

– Just A Lone
19 mins ago













the values are the same

– Just A Lone
18 mins ago





the values are the same

– Just A Lone
18 mins ago




1




1





If two variables refer to the same value between -5 and 256 (as opposed to use) then by definition there is only one object.

– Yusufsn
18 mins ago







If two variables refer to the same value between -5 and 256 (as opposed to use) then by definition there is only one object.

– Yusufsn
18 mins ago






1




1





@Yusufsn No. For bigger integers (>256) it's not true.

– ajnLJA-0184
13 mins ago





@Yusufsn No. For bigger integers (>256) it's not true.

– ajnLJA-0184
13 mins ago












4 Answers
4






active

oldest

votes


















7














Python uses shared small integers to help quick access. Integers range from [-5, 256] already exists in memory, so if you check the address, they are the same. However, for larger integers, it's not true.



a = 10e5
b = 10e5
a is b # False


Wait, what? If you check the address of the numbers, you'll find something interesting:



a = 1
b = 1
id(a) # 4463034512
id(b) # 4463034512

a = 257
b = 257
id(a) # 4642585200
id(b) # 4642585712


It's called integer cache. You can read more about the integer cache here: https://wsvincent.com/python-wat-integer-cache/




“The current implementation keeps an array of integer objects for all
integers between -5 and 256, when you create an int in that range you
actually just get back a reference to the existing object.”




Why? Because small integers are more frequently used by loops. Using reference to existing objects instead of creating a new object saves an overhead.






share|improve this answer










New contributor




ajnLJA-0184 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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    2














    If you take a look at Objects/longobject.c, which implements the int type for CPython, you will see that the numbers between -5 (NSMALLNEGINTS) and 256 (NSMALLPOSINTS - 1) are pre-allocated and cached. This is done to avoid the penalty of allocating multiple unnecessary objects for the most commonly used integers. This works because integers are immutable: you don't need multiple references to represent the same number.






    share|improve this answer































      0














      Python doesn't know anything until you tell it. So in your code above, when you initialize a and b, you are storing those values(in the register or RAM), and calling the place to store it a and b, so that you can reference them later. If you didn't initialize the variable first, python would just give you an error.






      share|improve this answer








      New contributor




      Monster AR44 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.





















      • I think you're missing the point of the question. a == b is obviously true. OP is asking why a is b is true.

        – Mad Physicist
        19 mins ago



















      0














      Why?



      is is kinda confusing:



      @ajnLJA-0184 is correct, but additionally, strings work well too, but when you do some operations to it, but still the same strings, nope:



      >>> a = 'python'
      >>> b = 'python'
      >>> a is b
      True
      >>> a = 'pytho' + 'n'
      >>> b = 'pythonn'[:-1]
      >>> a is b
      False
      >>>


      But strange enough, it is different for integers:



      >>> a = 1
      >>> b = 1
      >>> a is b
      True
      >>> a = 3*8
      >>> b = 4*6
      >>> a is b
      True
      >>>


      But as the link he gave, there you can see that:



      >>> a = 257
      >>> b = 257
      >>> a is b
      False
      >>>


      How to know if it is gonna be True or not?



      Well, here is when id comes in handy:



      Here you go, just type in id and two parens and say your varaible...:



      >>> a = 1
      >>> b = 1
      >>> id(a)
      1935522256
      >>> id(b)
      1935522256
      >>> a = 257 # or -6
      >>> b = 257 # or -6
      >>> id(a)
      935705330960
      >>> id(b)
      935705331216
      >>>





      share|improve this answer





















      • 1





        Why d is a equals False in the first example? That's new.

        – ajnLJA-0184
        2 mins ago











      • @ajnLJA-0 Because the strings aren't direct like 'python', they do an operation to get 'python', that's why.

        – U9-Forward
        54 secs ago














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      4 Answers
      4






      active

      oldest

      votes








      4 Answers
      4






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      7














      Python uses shared small integers to help quick access. Integers range from [-5, 256] already exists in memory, so if you check the address, they are the same. However, for larger integers, it's not true.



      a = 10e5
      b = 10e5
      a is b # False


      Wait, what? If you check the address of the numbers, you'll find something interesting:



      a = 1
      b = 1
      id(a) # 4463034512
      id(b) # 4463034512

      a = 257
      b = 257
      id(a) # 4642585200
      id(b) # 4642585712


      It's called integer cache. You can read more about the integer cache here: https://wsvincent.com/python-wat-integer-cache/




      “The current implementation keeps an array of integer objects for all
      integers between -5 and 256, when you create an int in that range you
      actually just get back a reference to the existing object.”




      Why? Because small integers are more frequently used by loops. Using reference to existing objects instead of creating a new object saves an overhead.






      share|improve this answer










      New contributor




      ajnLJA-0184 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.

























        7














        Python uses shared small integers to help quick access. Integers range from [-5, 256] already exists in memory, so if you check the address, they are the same. However, for larger integers, it's not true.



        a = 10e5
        b = 10e5
        a is b # False


        Wait, what? If you check the address of the numbers, you'll find something interesting:



        a = 1
        b = 1
        id(a) # 4463034512
        id(b) # 4463034512

        a = 257
        b = 257
        id(a) # 4642585200
        id(b) # 4642585712


        It's called integer cache. You can read more about the integer cache here: https://wsvincent.com/python-wat-integer-cache/




        “The current implementation keeps an array of integer objects for all
        integers between -5 and 256, when you create an int in that range you
        actually just get back a reference to the existing object.”




        Why? Because small integers are more frequently used by loops. Using reference to existing objects instead of creating a new object saves an overhead.






        share|improve this answer










        New contributor




        ajnLJA-0184 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
        Check out our Code of Conduct.























          7












          7








          7







          Python uses shared small integers to help quick access. Integers range from [-5, 256] already exists in memory, so if you check the address, they are the same. However, for larger integers, it's not true.



          a = 10e5
          b = 10e5
          a is b # False


          Wait, what? If you check the address of the numbers, you'll find something interesting:



          a = 1
          b = 1
          id(a) # 4463034512
          id(b) # 4463034512

          a = 257
          b = 257
          id(a) # 4642585200
          id(b) # 4642585712


          It's called integer cache. You can read more about the integer cache here: https://wsvincent.com/python-wat-integer-cache/




          “The current implementation keeps an array of integer objects for all
          integers between -5 and 256, when you create an int in that range you
          actually just get back a reference to the existing object.”




          Why? Because small integers are more frequently used by loops. Using reference to existing objects instead of creating a new object saves an overhead.






          share|improve this answer










          New contributor




          ajnLJA-0184 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.










          Python uses shared small integers to help quick access. Integers range from [-5, 256] already exists in memory, so if you check the address, they are the same. However, for larger integers, it's not true.



          a = 10e5
          b = 10e5
          a is b # False


          Wait, what? If you check the address of the numbers, you'll find something interesting:



          a = 1
          b = 1
          id(a) # 4463034512
          id(b) # 4463034512

          a = 257
          b = 257
          id(a) # 4642585200
          id(b) # 4642585712


          It's called integer cache. You can read more about the integer cache here: https://wsvincent.com/python-wat-integer-cache/




          “The current implementation keeps an array of integer objects for all
          integers between -5 and 256, when you create an int in that range you
          actually just get back a reference to the existing object.”




          Why? Because small integers are more frequently used by loops. Using reference to existing objects instead of creating a new object saves an overhead.







          share|improve this answer










          New contributor




          ajnLJA-0184 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          share|improve this answer



          share|improve this answer








          edited 14 mins ago





















          New contributor




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          answered 20 mins ago









          ajnLJA-0184ajnLJA-0184

          1884




          1884




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          New contributor





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              2














              If you take a look at Objects/longobject.c, which implements the int type for CPython, you will see that the numbers between -5 (NSMALLNEGINTS) and 256 (NSMALLPOSINTS - 1) are pre-allocated and cached. This is done to avoid the penalty of allocating multiple unnecessary objects for the most commonly used integers. This works because integers are immutable: you don't need multiple references to represent the same number.






              share|improve this answer




























                2














                If you take a look at Objects/longobject.c, which implements the int type for CPython, you will see that the numbers between -5 (NSMALLNEGINTS) and 256 (NSMALLPOSINTS - 1) are pre-allocated and cached. This is done to avoid the penalty of allocating multiple unnecessary objects for the most commonly used integers. This works because integers are immutable: you don't need multiple references to represent the same number.






                share|improve this answer


























                  2












                  2








                  2







                  If you take a look at Objects/longobject.c, which implements the int type for CPython, you will see that the numbers between -5 (NSMALLNEGINTS) and 256 (NSMALLPOSINTS - 1) are pre-allocated and cached. This is done to avoid the penalty of allocating multiple unnecessary objects for the most commonly used integers. This works because integers are immutable: you don't need multiple references to represent the same number.






                  share|improve this answer













                  If you take a look at Objects/longobject.c, which implements the int type for CPython, you will see that the numbers between -5 (NSMALLNEGINTS) and 256 (NSMALLPOSINTS - 1) are pre-allocated and cached. This is done to avoid the penalty of allocating multiple unnecessary objects for the most commonly used integers. This works because integers are immutable: you don't need multiple references to represent the same number.







                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered 14 mins ago









                  Mad PhysicistMad Physicist

                  39k1682113




                  39k1682113























                      0














                      Python doesn't know anything until you tell it. So in your code above, when you initialize a and b, you are storing those values(in the register or RAM), and calling the place to store it a and b, so that you can reference them later. If you didn't initialize the variable first, python would just give you an error.






                      share|improve this answer








                      New contributor




                      Monster AR44 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                      Check out our Code of Conduct.





















                      • I think you're missing the point of the question. a == b is obviously true. OP is asking why a is b is true.

                        – Mad Physicist
                        19 mins ago
















                      0














                      Python doesn't know anything until you tell it. So in your code above, when you initialize a and b, you are storing those values(in the register or RAM), and calling the place to store it a and b, so that you can reference them later. If you didn't initialize the variable first, python would just give you an error.






                      share|improve this answer








                      New contributor




                      Monster AR44 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                      Check out our Code of Conduct.





















                      • I think you're missing the point of the question. a == b is obviously true. OP is asking why a is b is true.

                        – Mad Physicist
                        19 mins ago














                      0












                      0








                      0







                      Python doesn't know anything until you tell it. So in your code above, when you initialize a and b, you are storing those values(in the register or RAM), and calling the place to store it a and b, so that you can reference them later. If you didn't initialize the variable first, python would just give you an error.






                      share|improve this answer








                      New contributor




                      Monster AR44 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                      Check out our Code of Conduct.










                      Python doesn't know anything until you tell it. So in your code above, when you initialize a and b, you are storing those values(in the register or RAM), and calling the place to store it a and b, so that you can reference them later. If you didn't initialize the variable first, python would just give you an error.







                      share|improve this answer








                      New contributor




                      Monster AR44 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                      Check out our Code of Conduct.









                      share|improve this answer



                      share|improve this answer






                      New contributor




                      Monster AR44 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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                      answered 20 mins ago









                      Monster AR44Monster AR44

                      111




                      111




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                      New contributor





                      Monster AR44 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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                      Check out our Code of Conduct.













                      • I think you're missing the point of the question. a == b is obviously true. OP is asking why a is b is true.

                        – Mad Physicist
                        19 mins ago



















                      • I think you're missing the point of the question. a == b is obviously true. OP is asking why a is b is true.

                        – Mad Physicist
                        19 mins ago

















                      I think you're missing the point of the question. a == b is obviously true. OP is asking why a is b is true.

                      – Mad Physicist
                      19 mins ago





                      I think you're missing the point of the question. a == b is obviously true. OP is asking why a is b is true.

                      – Mad Physicist
                      19 mins ago











                      0














                      Why?



                      is is kinda confusing:



                      @ajnLJA-0184 is correct, but additionally, strings work well too, but when you do some operations to it, but still the same strings, nope:



                      >>> a = 'python'
                      >>> b = 'python'
                      >>> a is b
                      True
                      >>> a = 'pytho' + 'n'
                      >>> b = 'pythonn'[:-1]
                      >>> a is b
                      False
                      >>>


                      But strange enough, it is different for integers:



                      >>> a = 1
                      >>> b = 1
                      >>> a is b
                      True
                      >>> a = 3*8
                      >>> b = 4*6
                      >>> a is b
                      True
                      >>>


                      But as the link he gave, there you can see that:



                      >>> a = 257
                      >>> b = 257
                      >>> a is b
                      False
                      >>>


                      How to know if it is gonna be True or not?



                      Well, here is when id comes in handy:



                      Here you go, just type in id and two parens and say your varaible...:



                      >>> a = 1
                      >>> b = 1
                      >>> id(a)
                      1935522256
                      >>> id(b)
                      1935522256
                      >>> a = 257 # or -6
                      >>> b = 257 # or -6
                      >>> id(a)
                      935705330960
                      >>> id(b)
                      935705331216
                      >>>





                      share|improve this answer





















                      • 1





                        Why d is a equals False in the first example? That's new.

                        – ajnLJA-0184
                        2 mins ago











                      • @ajnLJA-0 Because the strings aren't direct like 'python', they do an operation to get 'python', that's why.

                        – U9-Forward
                        54 secs ago


















                      0














                      Why?



                      is is kinda confusing:



                      @ajnLJA-0184 is correct, but additionally, strings work well too, but when you do some operations to it, but still the same strings, nope:



                      >>> a = 'python'
                      >>> b = 'python'
                      >>> a is b
                      True
                      >>> a = 'pytho' + 'n'
                      >>> b = 'pythonn'[:-1]
                      >>> a is b
                      False
                      >>>


                      But strange enough, it is different for integers:



                      >>> a = 1
                      >>> b = 1
                      >>> a is b
                      True
                      >>> a = 3*8
                      >>> b = 4*6
                      >>> a is b
                      True
                      >>>


                      But as the link he gave, there you can see that:



                      >>> a = 257
                      >>> b = 257
                      >>> a is b
                      False
                      >>>


                      How to know if it is gonna be True or not?



                      Well, here is when id comes in handy:



                      Here you go, just type in id and two parens and say your varaible...:



                      >>> a = 1
                      >>> b = 1
                      >>> id(a)
                      1935522256
                      >>> id(b)
                      1935522256
                      >>> a = 257 # or -6
                      >>> b = 257 # or -6
                      >>> id(a)
                      935705330960
                      >>> id(b)
                      935705331216
                      >>>





                      share|improve this answer





















                      • 1





                        Why d is a equals False in the first example? That's new.

                        – ajnLJA-0184
                        2 mins ago











                      • @ajnLJA-0 Because the strings aren't direct like 'python', they do an operation to get 'python', that's why.

                        – U9-Forward
                        54 secs ago
















                      0












                      0








                      0







                      Why?



                      is is kinda confusing:



                      @ajnLJA-0184 is correct, but additionally, strings work well too, but when you do some operations to it, but still the same strings, nope:



                      >>> a = 'python'
                      >>> b = 'python'
                      >>> a is b
                      True
                      >>> a = 'pytho' + 'n'
                      >>> b = 'pythonn'[:-1]
                      >>> a is b
                      False
                      >>>


                      But strange enough, it is different for integers:



                      >>> a = 1
                      >>> b = 1
                      >>> a is b
                      True
                      >>> a = 3*8
                      >>> b = 4*6
                      >>> a is b
                      True
                      >>>


                      But as the link he gave, there you can see that:



                      >>> a = 257
                      >>> b = 257
                      >>> a is b
                      False
                      >>>


                      How to know if it is gonna be True or not?



                      Well, here is when id comes in handy:



                      Here you go, just type in id and two parens and say your varaible...:



                      >>> a = 1
                      >>> b = 1
                      >>> id(a)
                      1935522256
                      >>> id(b)
                      1935522256
                      >>> a = 257 # or -6
                      >>> b = 257 # or -6
                      >>> id(a)
                      935705330960
                      >>> id(b)
                      935705331216
                      >>>





                      share|improve this answer















                      Why?



                      is is kinda confusing:



                      @ajnLJA-0184 is correct, but additionally, strings work well too, but when you do some operations to it, but still the same strings, nope:



                      >>> a = 'python'
                      >>> b = 'python'
                      >>> a is b
                      True
                      >>> a = 'pytho' + 'n'
                      >>> b = 'pythonn'[:-1]
                      >>> a is b
                      False
                      >>>


                      But strange enough, it is different for integers:



                      >>> a = 1
                      >>> b = 1
                      >>> a is b
                      True
                      >>> a = 3*8
                      >>> b = 4*6
                      >>> a is b
                      True
                      >>>


                      But as the link he gave, there you can see that:



                      >>> a = 257
                      >>> b = 257
                      >>> a is b
                      False
                      >>>


                      How to know if it is gonna be True or not?



                      Well, here is when id comes in handy:



                      Here you go, just type in id and two parens and say your varaible...:



                      >>> a = 1
                      >>> b = 1
                      >>> id(a)
                      1935522256
                      >>> id(b)
                      1935522256
                      >>> a = 257 # or -6
                      >>> b = 257 # or -6
                      >>> id(a)
                      935705330960
                      >>> id(b)
                      935705331216
                      >>>






                      share|improve this answer














                      share|improve this answer



                      share|improve this answer








                      edited 15 secs ago

























                      answered 10 mins ago









                      U9-ForwardU9-Forward

                      18.5k51744




                      18.5k51744








                      • 1





                        Why d is a equals False in the first example? That's new.

                        – ajnLJA-0184
                        2 mins ago











                      • @ajnLJA-0 Because the strings aren't direct like 'python', they do an operation to get 'python', that's why.

                        – U9-Forward
                        54 secs ago
















                      • 1





                        Why d is a equals False in the first example? That's new.

                        – ajnLJA-0184
                        2 mins ago











                      • @ajnLJA-0 Because the strings aren't direct like 'python', they do an operation to get 'python', that's why.

                        – U9-Forward
                        54 secs ago










                      1




                      1





                      Why d is a equals False in the first example? That's new.

                      – ajnLJA-0184
                      2 mins ago





                      Why d is a equals False in the first example? That's new.

                      – ajnLJA-0184
                      2 mins ago













                      @ajnLJA-0 Because the strings aren't direct like 'python', they do an operation to get 'python', that's why.

                      – U9-Forward
                      54 secs ago







                      @ajnLJA-0 Because the strings aren't direct like 'python', they do an operation to get 'python', that's why.

                      – U9-Forward
                      54 secs ago












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                      Just A Lone is a new contributor. Be nice, and check out our Code of Conduct.
















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