Python and Java - A Side by Side Comparison

Thanks to Skip Montanaro, Chris Lawrence, Donald McCarthy, Bengt Richter, and Christian Pohlmann for helpful feedback on earlier versions of this page.

APPENDIX: About static vs. dynamic typing, and strong vs. weak typing, of programming languages.

There is widespread confusion or disagreement about the meanings of the words static, dynamic, strong and weak when used to describe the type systems of programming languages. What follows is a description of the way (or at least one of the ways) these terms are most commonly used.

In a statically typed language, every variable name is bound both (1) to a type (at compile time, by means of a data declaration) and (2) to an object. The binding to an object is optional — if a name is not bound to an object, the name is said to be null. Once a variable name has been bound to a type (that is, declared) it can be bound (via an assignment statement) only to objects of that type; it cannot ever be bound to an object of a different type. An attempt to bind the name to an object of the wrong type will raise a type exception.

In a dynamically typed language, every variable name is (unless it is null) bound only to an object. Names are bound to objects at execution time by means of assignment statements, and it is possible to bind a name to objects of different types during the execution of the program.

Here is an example. In a statically-typed language, the following sequence of statements (which binds an integer object, then a string object, to the name employeeName) is illegal. If employeeName had been declared to be an int, then the second statement would be illegal; if it had been declared to be a String, then the first statement would be illegal. But in a dynamically-typed language this sequence of statements is perfectly fine.

employeeName = 9
employeeName = "Steve Ferg" 

Python is a dynamically-typed language. Java is a statically-typed language.

In a weakly typed language, variables can be implicitly coerced to unrelated types, whereas in a strongly typed language they cannot, and an explicit conversion is required. (Note that I said unrelated types. Most languages will allow implicit coercions between related types — for example, the addition of an integer and a float. By unrelated types I mean things like numbers and strings.) In a typical weakly typed language, the number 9 and the string "9" are interchangeable, and the following sequence of statements is legal.

a  = 9
b = "9"
c = concatenate(a, b)  // produces "99"
d = add(a, b)          // produces 18

In a strongly typed language, on the other hand, the last two statements would raise type exceptions. To avoid these exceptions, some kind of explicit type conversion would be necessary, like this.

a  = 9
b = "9"
c = concatenate(  str(a),  b)
d = add(a,  int(b)  )

Both Java and Python are strongly typed languages. Examples of weakly typed languages are Perl and Rexx.

A third distinction may be made between manifestly typed languages in which variable names must have explicit type declarations, and implictly typed languages in which this is not required. Most static languages are also manifestly typed (Java certainly is), but Frank Mitchell notes that some are not: "Haskell and the dialects of ML, for example, can infer the type of any variable based on the operations performed on it, with only occasional help from an explicit type."

 |