(version française)

CSC_52064 - TD 2 - Mini-Python interpreter (in Java)

The goal of this lab is to build an interpreter for a simple fragment of Python, called mini-Python. You don't have to know Python. A mini-Python file has the following structure:

# zero, one or several function definitions at the beginning of the file
def fibaux(a, b, k):
    if k == 0:
        return a
    else:
        return fibaux(b, a+b, k-1)

def fib(n):
    return fibaux(0, 1, n)

# then one or several statements at the end of the file
print("a few values of the Fibonacci sequence:")
for n in [0, 1, 11, 42]:
    print(fib(n))

More generally, a mini-Python file is composed of an optional list of function definitions, followed by a list of statements. Caveat: the last statement must be followed by a newline.

Statements are: assignment, conditional, loop (for), output with print, returning a value with return, and evaluation an expression. Expressions are: a constant (Boolean, integer, or string), access to a variable, building a list (with syntax [e1, e2, ..., en]), access to a list element (with syntax notation e[i]), function call, or one of the operations +, -, *, //, ==, <>, <, <=, >, >=, and, or and not.

We also consider three built-in functions: list(range(n)) builds the list [0, 1, 2, ..., n-1] and len(l) returns the length of list l. (We only use list and range jointly in this way.)

Code supplied

To help you building this interpreter, we provide the basic structure (as a set of Java files together with a Makefile).

Download this archive: mini-python-java.tar.gz

Once uncompressed (with tar zxvf mini-python.tar.gz), you get a directory mini-python-java/. (If you use Eclipse, you can run it in the directory where mini-python-java/ is, with the command eclipse -data ., then create a project mini-python-java and disable option Use default location.) This project contains a package mini_python with the following files:

Syntax.java	abstract syntax of mini-Python (complete)
Lexer.java, MyLexer.java	lexical analysis (complete)
parser.java, sym.java	parsing (complete)
Interp.java	interpreter (to be completed)
Main.java	main file (complete)
Makefile	to automate the build, if you don't use an IDE (complete)

The library java-cup-11a-runtime.jar, contained in sub-directory lib/, must be added to the project (with Java Build Path -> Libraries -> Add JARs). The code compiles but is incomplete. Places to be completed are indicated with throw new Todo().

The executable takes a mini-Python file on the command line, with suffix .py. When it is absent, file test.py is used.

Question 1. Arithmetic Expressions

For the moment, we only consider arithmetic expressions without variables. Complete the interpreter to evaluate such expressions. Test on the following file

print(1 + 2*3)
print((3*3 + 4*4) // 5)
print(10-3-4)

whose output must be

7
5
3

Division and modulus operations must signal a division by zero, by raising exception Error.

To test, simply edit file test.py and run Main.

Question 2. Boolean Expressions and Conditionals

Complete methods isTrue and isFalse in class Value, which respectively decide whether a value is true or false. In Python, the value None, the Boolean False, the integer 0, the empty string "" and the empty list [] are all considered false, and any other value is considered true.

Then complete the interpreter with Boolean constants, arithmetic comparison and operations and, or and not. In Python, comparison is structural. You can use the method compareTo that is provided in class Value.

Finally, complete the interpreter to handle the conditional (construction Sif).

Test on the following file

print(not True and 1//0==0)
print(1<2)
if False or True:
    print("ok")
else:
    print("oups")

whose output must be

False
True
ok

Question 3. Variables

To handle variables (global variables but also local variables and parameters) we are going to use an environment, which maps each known variable to its value. This table is the attribute vars of the class Interp.

Complete the interpreter so that we can access variables (case Eident). Accessing a variable that is not in the map must raise an error.

Similarly, complete the interpreter so that we can assign a value to a variable (method visit(Sassign s)). This time, the variable may or may not be in the table. In the former case, its value is modified. In the latter case, it is added.

Finally, complete the interpreter so that we can concatenate two strings with operation +.

Test on the following file

x = 41
x = x+1
print(x)
b = True and False
print(b)
s = "hello" + " world!"
print(s)

whose output must be

42
False
hello world!

Question 4. Functions

We now consider function definitions and calls. Functions are stored in the global hash table functions. Each function name is mapped to a pair consisting of the list of its parameters and its body (a statement). The code supplied is already filling this table with function definitions.

Then complete the interpreter to handle function calls. For a call f(e1,...,en) to a function f defined as def f(x1,...,xn): s, we have to build a new environment that maps each formal parameter xi to the value of ei. This means building a new object of type Interp. Then we interpret the statement s (the body of the function) in this new environment. The statement return is interpreted using the Java exception Return (already defined). If the execution terminates without an explicit return, the value None must be returned.

Test on the following file

def fact(n):
    if n <= 1: return 1
    return n * fact(n-1)

print(fact(10))

whose output must be

Question 5. Lists

Add support for lists. To do so, complete function expr so that we can concatenate two lists with operation +, to interpret calls to len (length of a list) and list(range(n)) (the list [0, 1, 2, ..., n-1]), and to interpret expressions [e1, e2, ..., en] and e1[e2].

Complete function stmt to interpret the assignment of a list element (class Sset).

Finally, complete function stmt to interpret the for loop. The statement for x in e: s successively assigns to the variable x the values of the list e, and executes the statement s for each of them. The expression e must be evaluated only once.

Test using the program at the beginning of this page. The output must be:

Question 6. Other tests

Positive and negative tests are provided. To run your interpreter on these tests, launch make tests.

solution

back to the main page