Is Python strongly typed

Question

I ve come across links that say Python is a strongly typed language   However  I thought in strongly typed languages you couldn t do this   bob   1 bob    bob    I thought a strongly typed language didn t accept type-changing at run-time  Maybe I ve got a wrong  or too simplistic  definition of strong weak types   So  is Python a strongly or weakly typed language

User · Answer

There are some important issues that I think all of the existing answers have missed.

Weak typing means allowing access to the underlying representation. In C, I can create a pointer to characters, then tell the compiler I want to use it as a pointer to integers:

char sz[] = "abcdefg";
int *i = (int *)sz;

On a little-endian platform with 32-bit integers, this makes i into an array of the numbers 0x64636261 and 0x00676665. In fact, you can even cast pointers themselves to integers (of the appropriate size):

intptr_t i = (intptr_t)&sz;

And of course this means I can overwrite memory anywhere in the system.*

char *spam = (char *)0x12345678
spam[0] = 0;

_{* Of course modern OS's use virtual memory and page protection so I can only overwrite my own process's memory, but there's nothing about C itself that offers such protection, as anyone who ever coded on, say, Classic Mac OS or Win16 can tell you.}

Traditional Lisp allowed similar kinds of hackery; on some platforms, double-word floats and cons cells were the same type, and you could just pass one to a function expecting the other and it would "work".

Most languages today aren't quite as weak as C and Lisp were, but many of them are still somewhat leaky. For example, any OO language that has an unchecked "downcast",* that's a type leak: you're essentially telling the compiler "I know I didn't give you enough information to know this is safe, but I'm pretty sure it is," when the whole point of a type system is that the compiler always has enough information to know what's safe.

_{* A checked downcast doesn't make the language's type system any weaker just because it moves the check to runtime. If it did, then subtype polymorphism (aka virtual or fully-dynamic function calls) would be the same violation of the type system, and I don't think anyone wants to say that.}

Very few "scripting" languages are weak in this sense. Even in Perl or Tcl, you can't take a string and just interpret its bytes as an integer.* But it's worth noting that in CPython (and similarly for many other interpreters for many languages), if you're really persistent, you can use ctypes to load up libpython, cast an object's id to a POINTER(Py_Object), and force the type system to leak. Whether this makes the type system weak or not depends on your use cases—if you're trying to implement an in-language restricted execution sandbox to ensure security, you do have to deal with these kinds of escapes…

_{* You can use a function like struct.unpack to read the bytes and build a new int out of "how C would represent these bytes", but that's obviously not leaky; even Haskell allows that.}

Meanwhile, implicit conversion is really a different thing from a weak or leaky type system.

Every language, even Haskell, has functions to, say, convert an integer to a string or a float. But some languages will do some of those conversions for you automatically—e.g., in C, if you call a function that wants a float, and you pass it in int, it gets converted for you. This can definitely lead to bugs with, e.g., unexpected overflows, but they're not the same kinds of bugs you get from a weak type system. And C isn't really being any weaker here; you can add an int and a float in Haskell, or even concatenate a float to a string, you just have to do it more explicitly.

And with dynamic languages, this is pretty murky. There's no such thing as "a function that wants a float" in Python or Perl. But there are overloaded functions that do different things with different types, and there's a strong intuitive sense that, e.g., adding a string to something else is "a function that wants a string". In that sense, Perl, Tcl, and JavaScript appear to do a lot of implicit conversions ("a" + 1 gives you "a1"), while Python does a lot fewer ("a" + 1 raises an exception, but 1.0 + 1 does give you 2.0*). It's just hard to put that sense into formal terms—why shouldn't there be a + that takes a string and an int, when there are obviously other functions, like indexing, that do?

_{* Actually, in modern Python, that can be explained in terms of OO subtyping, since isinstance(2, numbers.Real) is true. I don't think there's any sense in which 2 is an instance of the string type in Perl or JavaScript… although in Tcl, it actually is, since everything is an instance of string.}

Finally, there's another, completely orthogonal, definition of "strong" vs. "weak" typing, where "strong" means powerful/flexible/expressive.

For example, Haskell lets you define a type that's a number, a string, a list of this type, or a map from strings to this type, which is a perfectly way to represent anything that can be decoded from JSON. There's no way to define such a type in Java. But at least Java has parametric (generic) types, so you can write a function that takes a List of T and know that the elements are of type T; other languages, like early Java, forced you to use a List of Object and downcast. But at least Java lets you create new types with their own methods; C only lets you create structures. And BCPL didn't even have that. And so on down to assembly, where the only types are different bit lengths.

So, in that sense, Haskell's type system is stronger than modern Java's, which is stronger than earlier Java's, which is stronger than C's, which is stronger than BCPL's.

So, where does Python fit into that spectrum? That's a bit tricky. In many cases, duck typing allows you to simulate everything you can do in Haskell, and even some things you can't; sure, errors are caught at runtime instead of compile time, but they're still caught. However, there are cases where duck typing isn't sufficient. For example, in Haskell, you can tell that an empty list of ints is a list of ints, so you can decide that reducing + over that list should return 0*; in Python, an empty list is an empty list; there's no type information to help you decide what reducing + over it should do.

_{* In fact, Haskell doesn't let you do this; if you call the reduce function that doesn't take a start value on an empty list, you get an error. But its type system is powerful enough that you could make this work, and Python's isn't.}

User · Answer

A Python variable stores an untyped reference to the target object that represent the value     Any assignment operation means assigning the untyped reference to the assigned object -- i e  the object is shared via the original and the new  counted  references   The value type is bound to the target object  not to the reference value   The  strong  type checking is done when an operation with the value is performed  run time    In other words  variables  technically  have no type -- it does not make sense to think in terms of a variable type if one wants to be exact   But references are automatically dereferenced and we actually think in terms of the type of the target object

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I just discovered a superb concise way to memorize it      Dynamic static typed experssion  strongly weakly typed value

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TLDR  Python typing is Dynamic so you can change a string variable to an int x    somestring  x   50  Python typing is Strong so you can t merge types   foo    3 -- gt  TypeError  cannot concatenate  str  and  int  objects  In weakly-typed Javascript this happens      foo  3    foo3    Regarding Type Inference Java forces you to explicitly declare your object types int x   50   Kotlin uses inference to realize it s an int x   50  But because both languages use static types  x can t be changed from an int   Neither language would allow a dynamic change like x   50 x    now a string

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You are confusing  strongly typed  with  dynamically typed    I cannot change the type of 1 by adding the string  12   but I can choose what types I store in a variable and change that during the program s run time   The opposite of dynamic typing is static typing  the declaration of variable types doesn t change during the lifetime of a program  The opposite of strong typing is weak typing  the type of values can change during the lifetime of a program

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According to this wiki Python article Python is both dynamically and strongly typed  provides a good explanation too    Perhaps you are thinking about statically typed  languages where types  can not change during program execution and type checking occurs during compile time to detect possible errors   This SO question might be of interest  Dynamic type languages versus static type languages and this Wikipedia article on Type Systems provides more information

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The term  strong typing  does not have a definite definition   Therefore  the use of the term depends on with whom you re speaking   I do not consider any language  in which the type of a variable is not either explicitly declared  or statically typed to be strongly typed   Strong typing doesn t just preclude conversion  for example   automatically  converting from an integer to a string   It precludes assignment  i e   changing the type of a variable    If the following code compiles  interprets   the language is not strong-typed   Foo   1 Foo    1   In a strongly typed language  a programmer can  count on  a type   For example  if a programmer sees the declaration   UINT64 kZarkCount   and he or she knows that 20 lines later  kZarkCount is still a UINT64  as long as it occurs in the same block  - without having to examine intervening code

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class testme object           A test object         def   init   self           self y   0  def f aTestMe1  aTestMe2       return aTestMe1 y   aTestMe2 y     c   testme             get a variable to the class c x   10               add an attribute x inital value 10 c y   4                change the default attribute value of y to 4  t   testme              declare t to be an instance object of testme r   testme              declare r to be an instance object of testme  t y   6                 set t y to a number r y   7                 set r y to a number  print f r t             call function designed to operate on testme objects  r y    I am r y         redefine r y to be a string  print f r t            POW      not good       The above would create a nightmare of unmaintainable code in a large system over a long period time  Call it what you want  but the ability to  dynamically  change a variables type is just a bad idea

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It s already been answered a few times  but Python is a strongly typed language    gt  gt  gt  x   3  gt  gt  gt  y    4   gt  gt  gt  print x y  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  unsupported operand type s  for     int  and  str    The following in JavaScript   var x   3     var y    4  alert x   y    Produces  34    That s the difference between weak typing and strong typing  Weak types automatically try to convert from one type to another  depending on context  e g  Perl   Strong types never convert implicitly   Your confusion lies in a misunderstanding of how Python binds values to names  commonly referred to as variables    In Python  names have no types  so you can do things like   bob   1 bob    bob  bob    An Ex-Parrot     And names can be bound to anything    gt  gt  gt  def spam            print  Spam  spam  spam  spam        gt  gt  gt  spam on eggs   spam  gt  gt  gt  spam on eggs   Spam  spam  spam  spam   For further reading   https   en wikipedia org wiki Dynamic dispatch  and the slightly related but more advanced   http   effbot org zone call-by-object htm

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i think  this simple example should you explain the diffs between strong and dynamic typing    gt  gt  gt  tup     1   1   1   gt  gt  gt  for item in tup          type item       lt type  str  gt   lt type  int  gt   lt type  float  gt   gt  gt  gt    java   public static void main String   args            int i   1          i    1     will be error         i    0 1      will be error

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Python is strongly  dynamically typed    Strong typing means that the type of a value doesn t change in unexpected ways  A string containing only digits doesn t magically become a number  as may happen in Perl  Every change of type requires an explicit conversion  Dynamic typing means that runtime objects  values  have a type  as opposed to static typing where variables have a type    As for your example  bob   1 bob    bob    This works because the variable does not have a type  it can name any object  After bob 1  you ll find that type bob  returns int  but after bob  bob   it returns str   Note that type is a regular function  so it evaluates its argument  then returns the type of the value    Contrast this with older dialects of C  which were weakly  statically typed  so that pointers and integers were pretty much interchangeable   Modern ISO C requires conversions in many cases  but my compiler is still lenient about this by default    I must add that the strong vs  weak typing is more of a continuum than a boolean choice  C   has stronger typing than C  more conversions required   but the type system can be subverted by using pointer casts   The strength of the type system in a dynamic language such as Python is really determined by how its primitives and library functions respond to different types  E g     is overloaded so that it works on two numbers or two strings  but not a string and an number  This is a design choice made when   was implemented  but not really a necessity following from the language s semantics  In fact  when you overload   on a custom type  you can make it implicitly convert anything to a number   def to number x          Try to convert function argument to float-type object         try           return float x       except  TypeError  ValueError            return 0   class Foo      def   init   self  number            self number   number      def   add   self  other           return self number   to number other    Instance of class Foo can be added to other objects     gt  gt  gt  a   Foo 42   gt  gt  gt  a    1  43 0  gt  gt  gt  a   Foo 42  gt  gt  gt  a   1 43 0  gt  gt  gt  a   None 42   Observe that even though strongly typed Python is completely fine with adding objects of type int and float and returns an object of type float  e g   int 42    float 1  returns 43 0   On the other hand  due to the mismatch between types Haskell would complain if one tries the following  42    Integer     1    Float   This makes Haskell a strictly typed language  where types are entirely disjoint and only a controlled form of overloading is possible via type classes

[python] Is Python strongly typed?

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