Table of contents

1. What is Py2EO?
2. Quick Start
3. How to contribute
4. How to transpile Py to EO
5. Python syntax and tests coverage
6. Big project transpilation results
   • Parser tests
   • Django
   • CPython
7. Architecture and design
8. How do we project Python to EOLang
   • print
   • while and while-try-break
   • lambda
   • evaluation order
   • simple evaluation
   • assignment
   • evaluation
   • conditionals with if
   • conditionals with if-elif-else
   • while
   • for
   • function definition
   • not supported Python features

What is Py2EO?

This is a transpiler of Python to EOLANG. It translates Python code to EOLANG programming language.

This transpiler receives python code as input data. Then received code is simplified with several AST->AST passes. After successfull simplification EOLANG output is generated.

EOLANG code can be translated to java and executed or analyzed statically via Polystat analyzer.

Traspiler is a source-to-source translator, source-to-source compiler (S2S compiler), transcompiler, or transpiler is a type of translator that takes the source code of a program written in a programming language as its input and produces an equivalent source code in the same or a different programming language

Quick Start

Install Java 14, get Py2EO executable.

Then, start with a simple Python program in app.py file:

print("Hello world!")

Transpile it:

java -jar <path-to-py2eo-executable> app.eo

You should get app.eo containing (among a lot of system stuff):

[] > apply
    stdout (sprintf "%s\n" ((pystring "Hello world!").as-string))

For detailed instructions follow

How to contribute

Fork repository, make changes, send us a pull request. We will review your changes and apply them to the master branch shortly, provided they don't violate our quality standards. To avoid frustration, before sending us your pull request please run full Maven build:

mvn clean package

What's Next?

Test it now on your own examples following detailed instructions

Examine our ways to test it here

Explore requirements and architecture design here

Also note that you should use Maven 3.6.3 with Java 14 or Maven 3.8.4 with Java 17 (but there is no Maven 3.8 package in Ubuntu and no Java 14 package, so manual installation is needed anyway).

How to transpile Py to EO

Tested on Ubuntu (20.04+) and Windows (7+), but instructions are for Ubuntu

Install maven (sudo apt install maven) - it also installs default JDK (version 11 for now)

Install Java (JDK or JRE) version 14 (yes, exactly 14). For example you can download it here and unpack it:

cd ~

wget https://download.java.net/java/GA/jdk14.0.1/664493ef4a6946b186ff29eb326336a2/7/GPL/openjdk-14.0.1_linux-x64_bin.tar.gz

tar x -z < openjdk-14.0.1_linux-x64_bin.tar.gz

You can either use released transpiler executables or build it on your own:

Obtain Py2EO master branch sources via git clone https://github.com/polystat/py2eo.git (install git via sudo apt install git), or download zipped artifacts

Setup the PATH and JAVA_HOME variables, for example:

PATH="$PWD/jdk-14.0.1/bin/:$PATH"

export JAVA_HOME="$PWD/jdk-14.0.1/"

Check (e. g. via java -version) that version 14.* is used

Go to Py2EO root and run mvn clean package -DskipTests=true in the same command line runtime were you have set PATH and JAVA_HOME variables, if succeeded you will get transpiler/target/transpiler-${version_code}-SNAPSHOT-jar-with-dependencies.jar

Create test file with python code (e.g. sample_test.py in Py2EO root), for example with these contents:

def conditionalCheck2():
    a = 4
    b = 2

Run java -jar .\py2eo-${version_code}-SNAPSHOT-jar-with-dependencies.jar <path/to/python/file>, e. g:

java -jar .\py2eo-${version_code}-SNAPSHOT-jar-with-dependencies.jar sample_test.py

Check output .eo file in the directory with python code with the same name (e. g. sample_test.eo). Try using -o argument to specify output path and/or name if needed

Follow instructions on how to run the resulting eo code or analyze with Polystat

Additional arguments:

Option	Action
-h,--help	Display available options
-o <file>	Write output to
-X,--debug	Produce execution debug output
-v,--version	Print version information

You can also use yegor256/py2eo image for Docker:

$ docker run -v $(pwd):/eo yegor256/py2eo hello.py -o hello.eo

This command will translate hello.py in the current directory, saving the output to the hello.eo file.

Python syntax and tests coverage

For the parser and transpiler modules there are unit tests, located in parser/src/test/scala/org/polystat/py2eo/parser/ and transpiler/src/test/scala/org/polystat/py2eo/transpiler/ respectively.

You can see this in the CI. Go to Actions → Java CI. Select any workflow run, go to test job and checkout the Build with Maven step.

We have handwritten tests that are divided into groups by type: functional (also divided into groups by constructs in accordance with the language specification), integration tests (tests for the polystat analyzer), "negative" tests, etc.

Functional tests, 1600+ lines of code. A detailed description of the particular tests is given on a separate wiki page. All these tests go through a full cycle of stages: from generating EO to executing Java. Functional tests are grouped by folders corresponding to python syntax constructs we support or are going to support, so we have easy way to calculate overall coverage and test passes successfully state. Progress is shown in each release description.

Functional tests prefixed with eo_blocked_ are known to be blocked by bugs in EO. In particular, the test eo_blocked_nfbce is blocked by objectionary/eo#1249 , all others are blocked by objectionary/eo#1127 .

For now we support 100.00% of the determined python syntax subset and 100.00% are passed successefully

You can see this in the enabled tests counter CI. Go to Actions → Enabled tests counter. Select any workflow run and checkout the Run counter step.

To proof this (run all test and get statistics) on clean Ubuntu (20.04+):

Install maven (sudo apt install maven) - it also installs default JDK (version 11 for now)

Install Java (JDK or JRE) version 14 (yes, exactly 14). For example, you can download it here and unpack it:

cd ~

wget https://download.java.net/java/GA/jdk14.0.1/664493ef4a6946b186ff29eb326336a2/7/GPL/openjdk-14.0.1_linux-x64_bin.tar.gz

tar x -z < openjdk-14.0.1_linux-x64_bin.tar.gz

Obtain Py2EO master branch sources via git clone https://github.com/polystat/py2eo.git (install git via sudo apt install git).

Setup the PATH and JAVA_HOME variables, for example:

PATH="$PWD/jdk-14.0.1/bin/:$PATH"

export JAVA_HOME="$PWD/jdk-14.0.1/"

Check (e. g. via java -version) that version 14.* is used

Go to Py2EO root and run in the same command line runtime were you have set PATH and JAVA_HOME variables:

Run transpilation

mvn clean test -q

Resulting eo-files are located in py2eo/transpiler/src/test/resources/org/polystat/py2eo/transpiler/results.

Copy it to the runEO directory

cp transpiler/src/test/resources/org/polystat/py2eo/transpiler/results/*.eo ./transpiler/src/test/resources/org/polystat/py2eo/transpiler/runEO

Then copy the preface lib

cp -a transpiler/src/main/eo/preface ./transpiler/src/test/resources/org/polystat/py2eo/transpiler/runEO

And run EO compiler

cd ./transpiler/src/test/resources/org/polystat/py2eo/transpiler/runEO

mvn clean test

You will get detailed statistics in output.

Py2EO is capable of transpiling more than hundreds of thousands lines of python code

Parser tests

We tested the py2eo parser on CPython, python language implementation tests, version 3.8. For all tests (250,000+ lines of Python code), python source code is parsed and printed again, replacing the original one. Then we run CPython's integration test to verify that printed tests are still valid.

You can see this in the Integration Tests CI. Go to Actions → Integration Tests. Select any workflow run, go to the ParserPrinter job and checkout the Run integration tests step.

To proof this (parse all tests from CPython and launch make test on CPython) on clean Ubuntu (20.04+):

Install maven (sudo apt install maven) - it also installs default JDK (version 11 for now).

Install Java (JDK or JRE) version 14 (yes, exactly 14). For example you can download it here and unpack it:

cd ~

wget https://download.java.net/java/GA/jdk14.0.1/664493ef4a6946b186ff29eb326336a2/7/GPL/openjdk-14.0.1_linux-x64_bin.tar.gz

tar x -z < openjdk-14.0.1_linux-x64_bin.tar.gz

Obtain Py2EO master branch sources via git clone https://github.com/polystat/py2eo.git (install git via sudo apt install git).

Setup the PATH and JAVA_HOME variables, for example:

PATH="$PWD/jdk-14.0.1/bin/:$PATH"

export JAVA_HOME="$PWD/jdk-14.0.1/"

Check (e. g. via java -version) that version 14.* is used

Go to Py2EO root in the same command line runtime were you have set PATH and JAVA_HOME variables and run Py2EO build

mvn clean package -DskipTests=true

if succeeded you will get transpiler/target/transpiler-${version_code}-SNAPSHOT-jar-with-dependencies.jar.

To reprint python input tests and verify them afterwards run

mvn clean -Dit.test=ParserPrinterIT verify -B

You will get verification results in output.

Django

We tested it on Django, a popular Python web framework. For all .py files (every .py is considered as particular test) from Django repository (440,000+ lines of Python code) EO is generated and passes EO syntax check stage. Yet not tried to generate Java for this, since сompiling and execution of Java code obtained this way seems to be pointless.

You can see this in the Integration Tests CI. Go to Actions → Integration Tests. Select any workflow run, go to the Django job and checkout the Run integration tests step.

To proof this (transpile Django python source code and perform EO syntax verification) on clean Ubuntu (20.04+):

Install maven (sudo apt install maven) - it also installs default JDK (version 11 for now).

Install Java (JDK or JRE) version 14 (yes, exactly 14). For example you can download it here and unpack it:

cd ~

wget https://download.java.net/java/GA/jdk14.0.1/664493ef4a6946b186ff29eb326336a2/7/GPL/openjdk-14.0.1_linux-x64_bin.tar.gz

tar x -z < openjdk-14.0.1_linux-x64_bin.tar.gz

Obtain Py2EO master branch sources via git clone https://github.com/polystat/py2eo.git (install git via sudo apt install git).

Setup the PATH and JAVA_HOME variables, for example:

PATH="$PWD/jdk-14.0.1/bin/:$PATH"

export JAVA_HOME="$PWD/jdk-14.0.1/"

Check (e. g. via java -version) that version 14.* is used

Go to Py2EO root in the same command line runtime were you have set PATH and JAVA_HOME variables and run Py2EO build

mvn clean package -DskipTests=true

if succeeded you will get transpiler/target/transpiler-${version_code}-SNAPSHOT-jar-with-dependencies.jar.

To generate EO files and verify EO syntax afterwards run

mvn clean -Dit.test=DjangoIT verify -B

You will get EO source code in py2eo/transpiler/src/test/resources/org/polystat/py2eo/transpiler/django and verification (provided with EO) results in output.

CPython

Also, we tested Py2EO on CPython, python language implementation tests, version 3.8. For all tests (250,000+ lines of Python code), EO is generated and passes EO syntax check stage. Subsequent Java generation (and, therefore, Java compilation and execution), comes to Python runtime transpilation issue. Got plans to come back to issue after majority of functional "simple" tests will pass.

You can see this in the Integration Tests CI. Go to Actions → Integration Tests. Go to the CPython job, select any workflow run and checkout the Run integration tests step.

To proof this (transpile CPython tests source code and perform EO syntax verification) on clean Ubuntu (20.04+):

Install maven (sudo apt install maven) - it also installs default JDK (version 11 for now) Install gcc compiler*

Install Java (JDK or JRE) version 14 (yes, exactly 14). For example, you can download it here and unpack it:

cd ~

wget https://download.java.net/java/GA/jdk14.0.1/664493ef4a6946b186ff29eb326336a2/7/GPL/openjdk-14.0.1_linux-x64_bin.tar.gz

tar x -z < openjdk-14.0.1_linux-x64_bin.tar.gz

Obtain Py2EO master branch sources via git clone https://github.com/polystat/py2eo.git (install git via sudo apt install git).

Setup the PATH and JAVA_HOME variables, for example:

PATH="$PWD/jdk-14.0.1/bin/:$PATH"

export JAVA_HOME="$PWD/jdk-14.0.1/"

Check (e. g. via java -version) that version 14.* is used

Go to Py2EO root and in the same command line runtime were you have set PATH and JAVA_HOME variables run

mvn clean package -DskipTests=true

If succeeded you will get transpiler/target/transpiler-${version_code}-SNAPSHOT-jar-with-dependencies.jar.

To generate EO files and verify EO syntax afterwards run

mvn clean -Dit.test=CPythonIT verify -B

You will get EO source code in py2eo/transpiler/src/test/resources/org/polystat/py2eo/transpiler/testParserPrinter/afterParser/cpython and verification (provided with EO) results in output.

Also, we use Checker - a tool that reduces project testing time using input test mutations, as a part of test procedure . It's included in CI. Checkout more here.

Architecture and design

Py2EO meets the following requirements:

• The jar executable should take the path of the python input file as a command line argument, and optionally take the path of the output file
• If the Python input file has valid Python 3.9 syntax, translate it to eolang and write the result to the output file provided, or place the result near the input file if no output files were provided
• If the input python does not have valid Python 3.9 syntax, inform the user
• The repository should provide a set of tests which can be transpiled to the executable eolang code
• The repository should provide tools for transpiling some big python project, indicate that no exceptions were thrown from the transpiler and the resulting eolang files are syntactically correct

Py2EO architecture can be described as the following workflow:

• The main function parses the command line arguments and (if an existing input file was provided) reads the file
• The parser module uses the ANTLR parser to build an abstract syntax tree and then maps its tree to our own internal representation (also based on AST)
• The SimplePass module applies several python-to-python passes to eliminate some constructions which are not supported in eolang. These include:
  • Eliminate if-else statements with > 2 branches (rewrite an if-elsif-else as many if-else statements)
  • Eliminate for: rewrite it as a while + try
  • Prepend each identifier with x, so that no identifier starts with a capital letter
  • Standardize (simplify) quotes for string literals
  • Simplify except clauses for exception handling: change multiple except clauses of a try block to one except:, which catches everything, and a series of if-else, which emulates the behaviour of all the typed except clauses and reraises the exception if it should not be caught
  • Eliminate if-else again (because complex if-else statements appear as a result of the previous pass)
  • Eliminate complex assignments (i.e., translate a = b = c to b = c; a = b)
  • Eliminate simple cases of method calls (that is, substitute pointer to this explicitly)
  • Extract all function calls to the statement level to make the execution order explicit (i.e., translate a = f(1) + g(2) to tmp1 = f(1); tmp2 = g(2); a = tmp1 + tmp2
• The resulting simplified AST is then translated to the eolang code and printed to the provided output path or to the file next to the input file

Py2EO project consists of 3 modules: parser, checker, transpiler. All the modules have sample unit tests in them.

How do we project Python to EOLang

We analyzed python language and EOlang to determine the subset of Python features, corresponding restriction, design decisions that are explained within translation projections examples in this section.

Here we reference to the python language reference version 3.8.1 and are following the order of presentation prodosed there.

Let's start from classic "Hello, world!"

Print

print(x) is translated to stdout (sprintf "%s" (xx.as-string)) You may use this example:

x = 1
print(x)

or this

print("Hello, world!")

Comments, Indentation, Explicit and Implicit line joining, Whitespace between tokens (see sec 2) are supported by the parser. No additional support is needed, because these are just pecularities of the syntax.

A Python program is constructed from code blocks (see sec 4). A block is a piece of Python program text that is executed as a unit. Names refer to objects. Names are introduced by name binding operations. Dynamically adding/removing names is not supported. All the statically known names are implemented as a cage object of EO. This allows to implement assignments. EO objects are also visibility scopes for identifiers, so several variables with the same name in different scopes are implemented directly.

Exceptions, break, continue, return (see sec 4) are currently all implemented with the help of the goto object.

While

Consider this python code

while True: break

We translate it this way

goto
  [doBreak]
    TRUE.while
      doBreak.forward 0

While-try-break

Now consider this:

flag = 5
while True:
  try:
    break
  finally: flag = 10

From this section we know that "When a return, break or continue statement is executed in the try suite of a try…finally statement, the finally clause is also executed ‘on the way out.’". So, our implementation of try by means of goto must catch both exceptions and the break. It then must execute finally and rethrow everything that was not processed by the except clause (including break). The break then will be caught again up the stack by the implementation of while. Like so:

flag.write 5
goto
  [stackUp]
    TRUE.while
      write.
        resultOfTry
        goto
          [stackUp]
            stackUp.forward breakConstant // this is the break
      if.
        is-exception resultOfTry
        // here goes the implementation of except clause, which is BTW empty in our example
        0
      flag.write 10 // here goes the implementation of finally, so it is executed for exceptions and for the break
      if.
        is-break resultOfTry
        stackUp.forward resultOfTry  // redirect the break further up the stack
        0

We may also imagine a return that happens somewhere deep in the hierarchy of whiles and tries. This is a kind of semi-manual stack unwinding.

Complex, float and int numbers (see sec 6) are implemented as EO objects, which have a __class__ field, which is used to get a type of an object. Each arithmetic operation compares types of its operands and performs necessary conversion before evaluating the result.

Each identifier (see sec 6) is prepended with x because a python identifier may start with a capital letter, while one in EO cannot.

Integer, boolean, string and float literals (see sec 6) are wrapped in respective EO wrapper objects:

• 10 is translated to (pyint 10)
• "Hello, world" is translated to (pystring "Hello, world")

Try to translate print(10) or print("Hello, world"), for example.

Considering parenthesized forms (see sec 6). Almost every generated EO expression is parenthesized, but these parantheses are not related to the parantheses in the original python expressions.

Displays for lists, sets and dictionaries (see sec 6) are supported via a python-to-python pass: a display will be converted to a for loop.

Generator expressions (see sec 6) are a simplified form of coroutines and may contain yield, so they are not supported.

Yield expression (see sec 6) is a part of coroutines. We have no plans to support the coroutines right now.

Statically known attributes of python classes (see sec 6) are translated into attributes of the respective EO objects, so obj.attr is translated to obj.attr.

Subscriptions, slicing (see sec 6) are not yet implemented, but should be implemented with calling .__getitem__ and .__setitem__ methods of array objects. That is,

• a[i] should be translated to something like (a.__getitem__ i)
• and a[i] = x should be translated to something like (a.__setitem__ i x)

See the section on function definition for the examples on how to call (see sec 6) a function.

(see sec 6) is also a part of coroutines, no plans to support it.

Different arithmetics and logic binary and unary operations (see sec 6) are translated to calls of the respective functions of the EO standard library. Like, a == b is translated to (a.eq b), not x goes to (x.not) and so on.

Assignment expressions (see sec 6) are not yet supported.

Conditional expressions (see sec 6) are passed like the following: a if c else b -> (c).if (a) (b)

Add enough context when try this, for example:

a = 7 
b = 6
x = a if a < b else b
print(x)

An anonymous function/lambda expression (see sec 6) is extracted to a named function. This is not hard because complex expressions are split into simpler as described here). For example code f = lambda x: x * 10 is translated to something like:

def anonFun0(xx):
  e0 = xx * 10
  return e0

Then this python is translated to EO.

Lambda

Add enough context when try this, for example:

f = lambda x: x * 10
x = f(11)
print(x)

Expression lists (see sec 6) are not yet supported. Should be supported by explicitly constructing a tuple out of an expression list. A star sholud be implemented as a function, which unfolds an iterable object.

Evaluation order (see sec 6) is about laziness. Python is not lazy and the order of execution of a complex expression is documented. EO is lazy. Thus, each expression must be split into simple pieces and a series of statements must be generated, which force each piece in the correct order.

Evaluation order

For example, this x = (1 + 2) * f(3 + 4, 5) is translated to

  (e1).write (((pyint 1).add (pyint 2)))
  (e1).force
  ((e1).<)
  mkCopy (xf) > tmp1
  (lhs0).write (tmp1.copy)
  (e2).write (((pyint 3).add (pyint 4)))
  (e2).force
  ((e2).<)
  (lhs1).write ((pyint 5))
  (lhs1).force
  tmp.write (goto ((((lhs0)).apply ((e2)) ((lhs1))).@))
  (tmp.xclass.xid.neq (return.xclass.xid)).if (stackUp.forward tmp) 0
  (e3).write (tmp.result)
  ((e3).<)
  (e4).write (((e1).mul (e3)))
  (e4).force
  ((e4).<)
  mkCopy (e4) > tmp2
  (xx).write (tmp2.copy)

Function definition

Add enough context when try this, for example:

def f(a, b): return a + b
x = (1 + 2) * f(3 + 4, 5)
print(x)

Operator precedence feature (see sec 6) is supported by the parser. For example, for an expression 1 + 2 * 3 the parser generates a syntax tree like Add(1, Mult(2, 3)), not Mult(Add(1, 2), 3).

Simple evaluation

Try to translate, for example

x = 1 + 2 * 3
print(x)

Expressions statements (see sec 7) should be easy to get with but are not yet done.

Assignment

Assignment statements (see sec 7) are passed as follows:

• x is prepended to each variable name in order to support capital first letter of a name
• local variable names are statically extracted and declared as cage in the beginning of a generated output

Python

x = 1

EO

This is added at the start of the generated function

cage > xx

This is put at the appropriate place according to the execution order

(xx).write (1)

Evaluation

Python

x = x + 1

EO

Seems overly complicated, but this is a generalized variant of this hack. Possibly the part with dddata can be simplified.

seq > @
  [] > tmp1
    memory > dddata
    dddata.write (((xx).add 1)) > @
  (e0).write (tmp1.dddata)
  ((e0).<)
  mkCopy (e0) > tmp2
  (xx).write (tmp2.copy)
  123

where mkCopy looks like this

[x] > mkCopy
  x' > copy
  copy.< > @

You may try to run this example:

x = 1 
x = x + 2 * 3
print(x)

Assert (see sec 7) will be done as a python-to-python pass, which basically substitutes assert to raise AssertionException

Pass (see sec 7) is a statement, which does nothing.

Del (see sec 7) is an operator to delete attributes of objects dynamically. Not supported therefore.

Return (see sec 7) is discussed in function definition below.

Yield (see sec 7) is a part of coroutines. No plans to support the coroutines now.

Raise (see sec 7) is described in exceptions section before.

Break and Continue (see sec 7) is described in while section before. See the section on while.

Import (see sec 7) is not yet supported.

For global and Nonlocal (see sec 7) we need closure for full support.

If-elif-else (see sec 8) passes illustrated below:

Try adding print(x) at the end of examples below to run them).

Conditionals with if

Python

x = 1
if True:
  x = 2

EO

cage > tmp
cage > xx
seq > @
  (xx).write (1)
  TRUE.if
    seq
      (xx).write (2)
      TRUE
    seq
      TRUE
  123

Conditionals with if-elif-else

Python

x = 1
if True:
  x = 2
elif False:
  x = 3
else:
  x = 4

EO

cage > tmp
cage > xx
seq > @
  (xx).write (1)
  TRUE.if
    seq
      (xx).write (2)
      TRUE
    seq
      FALSE.if
        seq
          (xx).write (3)
          TRUE
        seq
          (xx).write (4)
          TRUE
      TRUE
  123

While

While (see sec 8) passes illustrated below:

Python

Imagine that the following code is a whole body of a function

x = 0
while True:
  x = x + 1

EO

Here, we have to support break with the help of goto, but also we should be able to pass other non-trivial returns like exceptions and return through the goto construct. This makes the code more complicated than just a direct use of while First, we declare x and some temporary variables

[stackUp]
  seq > @
    cage > tmp
    cage > xx
    cage > e0

    seq > @
      (xx).write (0)

Here, we wrap the while in a goto and store the result

      write.
        xcurrent-exception
        goto
          [stackUp]
            seq > @
              TRUE.while
                [unused]
                  seq > @

This part is just an overly generalized implementation of x = x + 1

                   [] > tmp1
                     memory > dddata
                     dddata.write (((xx).add 1)) > @
                   (e0).write (tmp1.dddata)
                   ((e0).<)
                   mkCopy (e0) > tmp2
                   (xx).write (tmp2.copy)
                   TRUE

Now, if we leave the while normally, we must still return something to be written to the current-exception variable, which is the raiseNothing object

             stackUp.forward raiseNothing

Next, if the goto result is not break, we basically rethrow it to the outer frame. Note, there are two stackUp objects here. The inner one is used to exit the while loop, while the outer one is used to exit the whole code block. It is only needed if the block is a body of another while or function or try block.

      if.
       xcurrent-exception.xclass.xid.neq (break.xclass.xid)
       stackUp.forward xcurrent-exception
       0

Try to run this example:

x = 1
while x < 100:
    x = 2 * x

print(x)

For

A for (see sec 8) loop over an iterator is transformed into a while inside a try:

Python

x = 0
for i in r: x = x + i

Python after for elimination

x = 0
it0 = r.__iter__()
try:
    while (True):
        i = it0.__next__()
        x = (x + i)
except StopIteration:
    pass

The resulting code is then transformed to EO.

Try to run this example

x = 0
for i in range(4):
    x = x + i

print(x)

Try (see sec 8) is passed as follows:

Python

  def f(a, b):
    try:
      return a
    finally:
      return b

EO

[] > f
  [xaNotCopied xbNotCopied] > apply
    [stackUp] > @
      cage > tmp
      xaNotCopied' > xa
      xbNotCopied' > xb
      seq > @
        stdout "xf\n"
        xa.<
        xb.<
        write.
          xcurrent-exception
          goto
            [stackUp]
              seq > @
                stackUp.forward (return (xa))
                stackUp.forward raiseNothing
        seq
          if.
            xcurrent-exception.xclass.xid.eq (raiseNothing.xclass.xid)
            seq
            0
          stackUp.forward (return (xb))
          (xcurrent-exception.xclass.xid.neq (raiseNothing.xclass.xid)).if (stackUp.forward xcurrent-exception) 0
        123

Also, try this example

try:
  1 // 0
except ZeroDivisionError:
  print("Hello, world")

With (see sec 8) is not yet implemented. The plan is to do it as a python-to-python pass according do the example here

Function definition

Function definition (see sec 8) is non-trivial part here is to allow a function body to both do a return and to throw exceptions, which are not caught inside the function. This necessity forces us to wrap a function body in an object to be called with the help of goto.

Function definition example

Python

def f(): return 5

EO

Here, apply subobject is used to pass parameters (none in this example), the inner anonymous object with parameter stackUp should be called with the goto.

The stackUp is used both

• to throw exceptions
• to return values normally, in which case they are wrapped into a return object, so that they can be differentiated from exceptions

[] > f
  [] > apply
    [stackUp] > @
      cage > tmp
      seq > @
        stackUp.forward (return 5)
        123

Function call example

Python

x = f()

EO

Here, we first actually call f, then we must check if it returned normally or with exception. The exception is rethrown. The call

tmp.write (goto ((((xf)).apply).@))

Check for exception

(tmp.xclass.xid.neq (return.xclass.xid)).if (stackUp.forward tmp) 0

Extract a result from the return object

(e0).write (tmp.result)
((e0).<)

Assign its copy to x.

mkCopy (e0) > tmp1
(xx).write (tmp1.copy)
123

Try this example

def f(a, b): return a + b
x = (1 + 2) * f(3 + 4, 5)
print(x)

A class (see sec 8) is basically its constructor, i.e., a function, which returns an object.

Python

class c:
  field = 1
o = c()
o.field = 2

EO

Here c has the same calling convention as other functions (it has a field apply and must be called with goto). The inner result is the object to be returned. It is returned through a cage, otherwise this code just hangs.

[] > c
  newUID.apply 0 > xid
  [] > apply
    [stackUp] > @
      cage > pResult
      [] > result
        cage > xfield
        xc > xclass
        seq > initFields
          xfield.write 1
      seq (result.initFields) (pResult.write result) (stackUp.forward (return pResult)) > @

This is how object o is created:

tmp.write (goto ((((xc)).apply).@))
(tmp.xclass.xid.neq (return.xclass.xid)).if (stackUp.forward tmp) 0
(e0).write (tmp.result)
((e0).<)
mkCopy (e0) > tmp1
(xo).write (tmp1.copy)

Field assignment is then straightforward:

((xo).xfield).write (2)

Try this example

class c:
  field = 1
o = c()
o.field = 2

x = o.field

print(x)

No plans to support coroutines (see sec 8).

Not supported Python features

1. Any kind of yield, also coroutines and generators (incl generator expressions) -- no support in EO
2. Threads, async, futures, await -- no support in EO
3. Dynamic features of python (dynamic creation/change/lookup/deletion of variables, creation of classes with metaclasses etc., dynamic features of import) -- using completely dynamic features would make the output EO not statically analyzable
4. Multiple inheritance -- not obvious how to do that for a general case, but with the EO delegation principle in mind
5. The majority of standard library -- it is mostly written in C, so even if we support all of the python syntax, it is still a problem to support the library without rewriting it manually.
6. Star expressions are mostly not supported -- possible, but not yet finished
7. Array slicing is partially supported -- possible, but not yet finished
8. The import system is partially supported -- possible, but not yet finished