ToyPython Parser

ToyPython is a minimal Python-like interpreter built with ANTLR4 and Python. It parses and interprets a small subset of Python syntax, including assignments, print statements, arithmetic, and if-blocks with indentation.

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What’s Included

• ToyPython.g4 – The ANTLR grammar for the language.
• ToyPythonLexer.py / ToyPythonParser.py / ToyPythonListener.py – ANTLR-generated Python files.
• ToyPythonSemanticListener.py – Custom Python code for interpreting ToyPython code.
• main.py – Preprocesses input for indentation, runs the parser, and executes the code.
• example.toy – An example ToyPython program.

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How It Works

1. Parsing:
   The grammar covers basic statements (assignment, print, if), simple expressions, and blocks using indentation.
2. Indentation:
   Python-like indentation is supported by preprocessing source code and custom lexer tweaks.
3. Interpreting:
   The ToyPythonSemanticListener.py walks the parse tree and actually runs the code (e.g., does assignments, prints values).

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Running It

1. Make sure you have Python 3 and install the ANTLR runtime:

   pip install antlr4-python3-runtime

2. (Only if you edit ToyPython.g4) Regenerate parser files:

   java -jar antlr-4.13.2-complete.jar -Dlanguage=Python3 ToyPython.g4

   This will generate (or update) files like:

   • ToyPythonLexer.py
   • ToyPythonParser.py
   • ToyPythonListener.py

3. Run the example:

   python main.py example.toy

   This will output the parse tree and interpret the program.

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Example Inputs & Outputs

Printing, Assignment, and If Statement

Input:

x = 5
print("Hello, world!")
if x > 0:
    print("x is positive")
print(x)

Output:

Parsed successfully!
Parse Tree:
(program (statement (simpleStatement x = (expression (term (factor 5))) \n)) (statement (simpleStatement print ( (expression (term (factor "Hello, world!"))) ) \n)) (statement (compoundStatement if (comparison (expression (term (factor x))) > (expression (term (factor 0)))) : \n <INDENT> (statement (simpleStatement print ( (expression (term (factor "x is positive"))) ) \n)) <DEDENT>)) (statement (simpleStatement print ( (expression (term (factor x))) ) \n)) <EOF>)
Hello, world!
x is positive
5

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String Assignment, String Concatenation, and Comparison

Input:

name = "Alice"
score = 10
if score >= 10:
    print("Congratulations " + name)
print("End of program")

Output:

Parsed successfully!
Parse Tree:
(program (statement (simpleStatement name = (expression (term (factor "Alice"))) \n)) (statement (simpleStatement score = (expression (term (factor 10))) \n)) (statement (compoundStatement if (comparison (expression (term (factor score))) >= (expression (term (factor 10)))) : \n <INDENT> (statement (simpleStatement print ( (expression (term (factor "Congratulations ") + (term (factor name)))) ) \n)) <DEDENT>)) (statement (simpleStatement print ( (expression (term (factor "End of program"))) ) \n)) <EOF>)
Congratulations Alice
End of program

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Arithmetic Expressions and Comparison

Input:

a = 3
b = 7
print(a + b)
if a < b:
    print("a is less than b")
print("Finished")

Output:

Parsed successfully!
Parse Tree:
(program (statement (simpleStatement a = (expression (term (factor 3))) \n)) (statement (simpleStatement b = (expression (term (factor 7))) \n)) (statement (simpleStatement print ( (expression (term (factor a) + (term (factor b)))) ) \n)) (statement (compoundStatement if (comparison (expression (term (factor a))) < (expression (term (factor b)))) : \n <INDENT> (statement (simpleStatement print ( (expression (term (factor "a is less than b"))) ) \n)) <DEDENT>)) (statement (simpleStatement print ( (expression (term (factor "Finished"))) ) \n)) <EOF>)
10
a is less than b
Finished

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If Condition That Fails

Input:

x = 0
if x != 0:
    print("Nonzero")
print("Done")

Output:

Parsed successfully!
Parse Tree:
(program (statement (simpleStatement x = (expression (term (factor 0))) \n)) (statement (compoundStatement if (comparison (expression (term (factor x))) != (expression (term (factor 0)))) : \n <INDENT> (statement (simpleStatement print ( (expression (term (factor "Nonzero"))) ) \n)) <DEDENT>)) (statement (simpleStatement print ( (expression (term (factor "Done"))) ) \n)) <EOF>)
Done

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Extending

• To add more language features, we would need to edit ToyPython.g4 and update ToyPythonSemanticListener.py.
• If we add grammar rules, re-run ANTLR as above.

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That’s it! Have fun experimenting with tiny Python.