3. Special facilities

3.1. Transcrypt's module mechanism

Transcrypt's module mechanism looks a lot like Python's but there are a few differences. Firstly, in Transcrypt it is good practice to use url-based unique module identifiers, e.g.

  • com.github.<my name>.<my module name>
  • org.python.pypi.<my module name>
  • com.<my company name>.<my module name>

To achieve optimal CPython compatibility, an exception is made for modules that are part of the CPython distribution, e.g. math.

Note that Transcrypt is meant be to used with JavaScript rather than Python libraries, since its focus isn't on the desktop but on the browser. Nevertheless a growing set of standard Python modules is part of the distribution, currently cmat, datetime, itertools, logging, math, random (partially), re (almost complete), time, turtle and warnings. Other modules are in the making.

Finding the code for a module proceeds as follows:

Suppose you import a module all.the.kings.men. Then the following paths will be searched respectively:

  • <directory of your main module>/all/the/kings/men.py
  • <directory of your main module>/all/the/kings/__javascript__/men.mod.js
  • <directory of your main module>/all/the/kings/men/__init__py
  • <directory of your main module>/all/the/kings/men/__javascript__/__init__.mod.js
  • transcrypt/Transcrypt/modules/all/the/kings/men.py
  • transcrypt/Transcrypt/modules/all/the/kings/__javascript__/men.mod.js
  • transcrypt/Transcrypt/modules/all/the/kings/men/__init__py
  • transcrypt/Transcrypt/modules/all/the/kings/men/__javascript__/__init__.mod.js
  • <CPython packages directory 1>/all/the/kings/men.py
  • <CPython packages directory 1>/all/the/kings/__javascript__/men.mod.js
  • <CPython packages directory 1>/all/the/kings/men/__init__py
  • <CPython packages directory 1>/all/the/kings/men/__javascript__/__init__.mod.js
  • <CPython packages directory 2>/all/the/kings/men.py
  • <CPython packages directory 2>/all/the/kings/__javascript__/men.mod.js
  • <CPython packages directory 2>/all/the/kings/men/__init__py
  • <CPython packages directory 2>/all/the/kings/men/__javascript__/__init__.mod.js
  • More CPython packages directories

As can be seen from the above list, modules local to your project take precedence of modules available in Transcrypt, which again take precedence over modules available globally in CPython. Note that even if modules are made available globally in CPython, importing them in Transcrypt gives special possibilities and restrictions. They are allowed to be written in native JavaScript, in which case they reside in the __javascript__ subdirectory of the module. They should not depend on C, C++ or features that are outside Python subset supported by Transcrypt.

Although under these guidelines it's quite possible to write modules that are importable both by CPyton and Transcrypt, most Transcrypt modules will be come from the JavaScript, rather than from the Python ecosystem. If both a Python and a JavaScript incarnation of a module are present, the Python module is only recompiled if it's younger than the corresponding JavaScript module, unless the -b switch is used.

Furthermore, note that the __init__.py or __init__.mod.js file of a module is executed if and only if that module is imported, not if it's just somewhere in the hierarchy of directories containing that module. Furthermore the global code of a module is executed only once, no matter how often that module is imported, as is equally the case with CPython.

Another deviation from CPython is that the inclusion of a module has to be decided upon compiletime. This means that imports cannot be runtime conditional, so e.g. cannot be under an if. For compiletime conditional imports you can use __pragma__ ('ifdef'). Also, since imports are resolved in one pass, cyclic imports are not supported.

As a consequence of the above, modules may be distributed as Python .py files, but also as JavaScript-only .mod.js files. The JavaScript files may be hand written or generated by any tool including Transcrypt. Although they may be distributed in minified form, don't give them the extension .mod.min.js, that exension is reserved for minification by Transcrypt itself. Since Trancrypt will minify your files anyhow, you're encouraged to distribute the original, human readable sources, to facilitate debugging by users of your library.

3.2. Transcrypt's unit mechanism and creating native JavaScript component frameworks

It is possible to create native JavaScript component frameworks using Transcrypt. Components can be freely mixed on a page without requiring recompilation. A developer doesn't need Transcrypt or any knowledge of it to use such a framework. Each page has one unit that contains the Transcrypt runtime and, if needed, any central facilities. It is compiled with the -u .run switch. In addition each page can hold as many separate components as needed. Compile these with the -u .com switch. A loader is generated, accepting a list of components. By varying this list, each page can have it's own subset of components, without the need to recompile. The generated JavaScript code for components can be very small, typically a few hundred bytes. The runtime and central facilities are included in the page only once.

Note that the concepts of a unit and a module are orthogonal. All units of a framework share the same namespace. Modules, on the other hand, constitute a distinct namespace. An example of using a combination of units and modules is the following:

animals.html - the webpage that invokes the loader to load the runtime unit and certain components of the animals framework
        <script src = "__javascript__/animals_loader.js"></script>
            animals_loader ([
        <style>button {width:200px;}</style>
        <div id = "Felix">...</div>
        <button onclick="animals.Cat ('Felix');">Create cat Felix</button><br/>
        <button onclick="animals.find ('Felix') .feed ();">Feed cat Felix</button><br/>
        <button onclick="animals.find ('Felix') .greet ();">Greet cat Felix</button><br/>
        <div id = "Tiger">...</div>
        <button onclick="animals.Cat ('Tiger');">Create cat Tiger</button><br>
        <button onclick="animals.find ('Tiger') .feed ();">Feed cat Tiger</button><br/>
        <button onclick="animals.find ('Tiger') .greet ();">Greet cat Tiger</button><br/>
        <div id = "Pluto">...</div>
        <button onclick="animals.Dog ('Pluto');">Create dog Pluto</button><br>
        <button onclick="animals.find ('Pluto') .feed ();">Feed dog Pluto</button><br/>
        <button onclick="animals.find ('Pluto') .greet ();">Greet dog Pluto</button><br/>
animals_submodule.py - imported in the runtime unit
def getTaxoTag ():
    return ('animal')
animals.py - the runtime unit
import animals_submodule as asm

_individuals = {}

def find (name):
    return _individuals [name]

class Animal:   
    def __init__ (self, name, food, sound):
        _individuals [name] = self
        self.name = name
        self.food = food
        self.sound = sound
        self.fed = False
        document.getElementById (self.name) .innerHTML  = self.speak (f'I was born just now! My kingdom is: {asm.getTaxoTag ()}. My species is {self.species}')       
    def speak (self, text):
        return f'{self.name} says: ' + text

    def feed (self):
        document.getElementById (self.name) .innerHTML = self.speak (
                f'No thanks, I first want to greet you with {self.sound}!'
            if self.fed else
                f'Thanks a lot, I am now eating {self.food}!'
        self.fed = True
    def greet (self):
        document.getElementById (self.name) .innerHTML = self.speak (
                f'{self.sound}, {self.sound}, {self.sound}!'
            if self.fed else
                f'Sorry, I want to eat {self.food} first!'
        self.fed = False
cats_submodule.py - imported in the first component
def getTaxoTag ():
    return ('cat')
cats.py - the first component
import cats_submodule as csm

class Cat (Animal):
    def __init__ (self, name):
        self.species = csm.getTaxoTag ()
        super () .__init__ (name, 'fish', 'mraaaw')
dogs.py - imported in the second component
def getTaxoTag ():
    return ('dog')
dogs_submodule.py - the second component
import dogs_submodule as dsm

class Dog (Animal):
    def __init__ (self, name):
        self.species = dsm.getTaxoTag ()        
        super () .__init__ (name, 'meat', 'wooof')

To utilize minification, use .min.js files intead of .js files in your page. Since corresponding names have to be used in all units, minification is less effective than without the use of units.

3.3. Using browser stubs to test non-GUI code that uses console.log and window.alert

To test the non-GUI part of your code in a desktop rather than a browser environment, use from org.transcrypt.stubs.browser import *. This will allow you to call the window.alert and console.log functions in your code when you run it from the command prompt, using the -r command line switch: transcrypt -r <my main file name>. This will invoke CPython, searching the appropriate module paths as compilation would have done.

3.4. Creating JavaScript objects with __new__ (<constructor call>)

While creating objects in Transcrypt mostly happens in the plain Python way, e.g. canvas = Canvas (<parameters>), objects from 3rd party JavaScript libraries often have to be created using new. In Transcrypt such objects are created by calling the __new__ function, e.g. canvas = __new__ (Canvas (<parameters>), as can be seen in the Pong example. This mechanism is simple, follows Python's syntax rules and doesn't require updating of an encapsulation layer if a later version of the underlying JavaScript library features additional constructor functions. Therefore in most cases it is the preferred way to create objects who's initialization requires calling 3rd party JavaScript constructor functions.

As an alternative, instantiation and construction can be encapsulated in one function call. Since this in fact creates an alternative API facade for the 3rd party JavaScript library, such an encapsulation should be kept separate from the original library, e.g. by putting it in a separate importable module. The JavaScript code for this encapsulation would e.g. be <facade module name>.Canvas = function (<parameters>) {return new Canvas (<parameters>);};. After importing the facade module, canvas creation is straightforward: canvas = Canvas (<parameters>).

As a third alternative, encapsulation can be done in Python rather than JavaScript: def Canvas (<parameters>): return __new__ (<3rd party module name>.Canvas (<parameters>). Also in this case the creation syntax is simple: canvas = Canvas (<parameters>).

3.5. The __pragma__ mechanism

Pragma's are directives in the source code, that locally alter the behaviour of the compiler. Pragma's come in three varieties.

3.5.1. The function-like variety

__pragma__ (<parameters>)

is acceptable only to Transcrypt, CPython requires a stub with parameter *args. Such a stub can either be defined locally or imported:

from org.transcrypt.stubs.browser import __pragma__

3.5.2. The comment-like variety

# __pragma__ (<parameters>)

is acceptable both to Transcrypt and CPython. In CPython it does nothing.

N.B. Both varieties above have to be properly indented, matching indentation of their context.

3.5.3. The single line activation variety

<line of code> # __:<single parameter>

It will switch a facility on or off just for the line of code it's part of. Single line pragma's can only be used for pragma's with have a single parameter, it's name.

For example the following line:

vector2 = vector0 + vector1 # __:opov

will be compiled identially to:

__pragma__ ('opov'); vector2 = vector0 + vector1; __pragma__ ('noopov')

3.5.4. The single line deactivation variety

scalar2 = scalar0 + scalar1 # __:noopov

will be compiled identially to:

__pragma__ ('noopov'); scalar2 = scalar0 + scalar1; __pragma__ ('opov')

3.6. Identifier aliasing: __pragma__ ('alias', ...) and __pragma__ ('noalias', ...)

Calling __pragma__ ('alias', <Python id part>, <JavaScript id part>) at the start of a module will replace identifiers or parts thereof like in the following examples:

Example 1:

Used at the start of the module: __pragma__ ('alias', 'S', $)
Original in Python: Alias in JavaScript:
S ('body') $ ('body')
S__body $body
S__She__S__Sells__S__Sea__S__Shells__S $She$Sells$Sea$Shells$

Using the above alias, a piece of jQuery code will look like this in Python:

Use of __pragma__ ('alias', 'S', '$') in jquery_demo.py
__pragma__ ('alias', 'S', '$')

def start ():
    def changeColors ():
        for div in S__divs:
            S (div) .css ({
                'color': 'rgb({},{},{})'.format (* [int (256 * Math.random ()) for i in range (3)]),

    S__divs = S ('div')
    changeColors ()
    window.setInterval (changeColors, 500)

Example 2:

Used at the start of the module __pragma__ ('alias', 'jq', '$')
jq__body = jq (body) $body = $ ('body')

Note that the generated JavaScript only the modified identifiers will be present, not the original ones. So the JavaScript identifiers are only aliases for the Python ones, not for any identifier in the JavaScript code itself.

A number of aliases are predefined in the source code of Transcrypt as follows:

Transcrypt's predefined aliases

            # Format: ('<Python source identifier>', '<JavaScript target identifier>')

                                                    ('js_and', 'and'),
            ('arguments', 'py_arguments'),          ('js_arguments', 'arguments'),
            ('case', 'py_case'),
            ('clear', 'py_clear'),                  ('js_clear', 'clear'),
                                                    ('js_conjugate', 'conjugate'),
            ('default', 'py_default'),
            ('del', 'py_del'),                      ('js_del', 'del'),
            ('false', 'py_false'),
                                                    ('js_from', 'from'),
            ('get', 'py_get'),                      ('js_get', 'get'),
                                                    ('js_global', 'global'),
            ('Infinity', 'py_Infinity'),            ('js_Infinity', 'Infinity'),
            ('isNaN', 'py_isNaN'),                  ('js_isNaN', 'isNaN'),
            ('iter', 'py_iter'),                    ('js_iter', 'iter'),
            ('items', 'py_items'),                  ('js_items', 'items'),
            ('keys', 'py_keys'),                    ('js_keys', 'keys'),
            ('name', 'py_name'),                    ('js_name', 'name'),
            ('NaN', 'py_NaN'),                      ('js_NaN', 'NaN'),
            ('new', 'py_new'),
            ('next', 'py_next'),                    ('js_next', 'next'),
                                                    ('js_not', 'not'),
                                                    ('js_or', 'or'),
            ('pop', 'py_pop'),                      ('js_pop', 'pop'),
            ('popitem', 'py_popitem'),              ('js_popitem', 'popitem'),
            ('replace', 'py_replace'),              ('js_replace', 'replace'),
            ('selector', 'py_selector'),            ('js_selector', 'selector'),
            ('sort', 'py_sort'),                    ('js_sort', 'sort'),
            ('split', 'py_split'),                  ('js_split', 'split'),
            ('switch', 'py_switch'),
            ('type', 'py_metatype'),                ('js_type', 'type'),    # Only for the type metaclass, the type operator is dealth with separately in visit_Call
            ('TypeError', 'py_TypeError'),          ('js_TypeError', 'TypeError'),
            ('update', 'py_update'),                ('js_update', 'update'),
            ('values', 'py_values'),                ('js_values', 'values'),
            ('reversed', 'py_reversed'),            ('js_reversed', 'reversed'),
            ('setdefault', 'py_setdefault'),        ('js_setdefault', 'setdefault'),
                                                    ('js_super', 'super'),
            ('true', 'py_true'),
            ('undefined', 'py_undefined'),          ('js_undefined', 'undefined'),

The predefined aliases are used to resolve name conflicts between Python and JavaScript. Calling e.g <object>.sort (<params>) from Python will invoke a sort method with Python semantics, which in the generated JavaScript is called py_sort. Calling <object>.js_sort <params> from Python will invoke a sort method with JavaScript semantics, which in the generated JavaScript is simply called sort, as 3rd party JavaScript libraries will expect.

Calling __pragma__ ('noalias', <Python id part>) will remove the alias. Calling __pragma__ ('noalias') without second parameter will remove all aliases. WARNING: This will also remove the predefined aliases.

The alias mechanism is vital to both the compactness and speed of code generated by Transcrypt and to its seamless integration with JavaScript. It allows Transcrypt to extend native JavaScript objects without causing name conflicts even when JavaScript gets expanded over the years, but also without any conversions or other overhead. That's why in Transcrypt this approach is favored over wrapping native objects.

3.7. Generating __doc__ attributes from docstrings: __pragma__ ('docat') and __pragma__ ('nodocat')

Even though docstrings may be present in your code, by default __doc__ attributes are not generated, to keep the generated JavaScript code lean. You can switch on and off generation of __doc__ attributes for modules, classes, methods and functions with __pragma__ ('docat') and __pragma__ ('nodocat') respectively. There's also a -d / --docat command line switch, which can be overruled per module by __pragma__ ('nodocat'). Advice: Though these pragma's are flexible, its advisable to use them only at the spot indicated in the docstrings testlet, to retain CPython compatibility. Note that __pragma__ ('docat') follows rather than precedes the module docstring, since CPython dictates the module docstring to be the first statement in a module.

3.8. Surpassing the speed of native JavaScript: __pragma__ ('fcall') and __pragma ('nofcall')

Fast calls or fcalls are method calls where the method isn't an attribute of an object's prototype, but of the object itself, even if this method is inherited over multiple levels. This means that only for the first call the prototype chain is searched and the method is bound to the object. All subsequent calls invoke the bound method directly on the object. You can use the -f command line switch to generate fcall's pervasively, making your objects slightly larger since they will contain references to their methods. If you don't want that, just place the definition of the method(s) or class(es) you wish to optimize between __pragma__ ('fcall') and __pragma__ ('nofcall'). You can also do the opposite, using the -f switch and exempting some method(s) or class(es) by embedding them between __pragma__ ('nofcall') and __pragma__ ('fcall').

Note that these pragmas have to be applied on the function definition location rather than the call location. Placing __pragma__ ('fcall') or __pragma__ ('nofcall') at the beginning of a module will influence all methods defined in that module. The fcall mechanism is a form of memoization and one example of a transpiler being able to generate optimized code that surpasses common hand coding practice. The fcall mechanism influences neither the pure Python syntax nor the semantics of your program.

3.9. Enabling Pythons send syntax: __pragma__ ('gsend') and __pragma ('nogsend')

These pragmas enable respectively disable use of Pythons send syntax for generators, which is disabled by default. An example of its use is in the following code:

Use of __pragma ('gsend') to enable Pythons send syntax fo generators
    __pragma__ ('gsend')

    def test0 ():
        r = 0
        while True:
            r = r + (yield r)

    gen0 = test0()
    next (gen0)
    autoTester.check (gen0.send (1))
    autoTester.check (gen0.send (2))

    def test1 ():
        r = 0
        while True:
            r = (yield r) + r

    gen1 = test1()
    next (gen1)
    autoTester.check (gen1.send (3))
    autoTester.check (gen1.send (4))

3.10. Automatic conversion to iterable: __pragma__ ('iconv') and __pragma__ ('noiconv')

In CPython sometimes automatic conversion from a non-iterable to an iterable type takes place. This comes at the expense of a runtime typecheck and is by default avoided in Transcrypt for that reason. Iteration through the keys of a dict without explicitly calling its keys () member is a frequent use case of automatic conversion. To switch on automatic conversion for dicts locally, __pragma__ ('iconv') and __pragma__ ('noiconv') can be used. The alternative is to switch on automatic conversion globally using the -i command line switch. Use of this switch is disadvised, especially for numerical processing code containing nested loops, since it adds the mentioned typecheck to each for .. in .. loop. When designing numerical processing libraries, it's advisable to use __pragma__ ('noiconv') explicitly at the start of each performance-sensitive module. The result will be that even when an application developer chooses to use the -i switch, the performance of the computations won't suffer.

3.11. Conditional compilation: __pragma__ ('ifdef', <symbol>), __pragma__ ('ifndef', <symbol>), __pragma__ ('else') and __pragma__ ('endif')

A piece of code in between __pragma__ ('ifdef', <symbol>) and __pragma__ ('endif') will only be compiled if <symbol> occurs in the global list of defined symbols.

This pragma works mainly in combination with the -s / --symbols <names joined by $> command line option. On top of that, some command line options automatically add symbols, without using the -s switch. An example is -e 6 option, which adds the symbol e6 to the global list of defined symbols.

An example of the use of this pragma is found the autotest code below:

Use of conditional compilation to prevent Transcrypt code that requires JavaScript 6 from being compiled in the JavaScript 5 mode.
import org.transcrypt.autotester

import arguments
import attribs_by_name
import builtin_super
import byte_arrays
import callable_test
import classes
import complex_numbers
import conditional_expressions
import control_structures

# __pragma__ ('ifdef', '__py3.6__') # Needed because Transcrypt imports are compile time
if '__py3.6__' in __symbols__:      # Needed because CPython doesn't understand pragma's
    import dashed_numbers
# __pragma__ ('endif')

import data_structures
import decorators
import dict_comprehensions
import dictionaries
import div_issues
import div_pulls
import docstrings
import exceptions
import extended_slices

# __pragma__ ('ifdef', '__py3.6__') # Needed because Transcrypt imports are compile time
if '__py3.6__' in __symbols__:      # Needed because CPython doesn't understand pragma's
    import fstrings
# __pragma__ ('endif')

import general_functions
import globals_function
import indices_and_slices

# __pragma__ ('ifdef', '__esv6__')
if '__esv6__' in __symbols__:
    import iterators_and_generators
# __pragma__ ('endif')

import lambda_functions
import list_comprehensions
import local_classes
import metaclasses
import method_and_class_decorators
import module_builtin
import module_cmath

# __pragma__ ('ifdef', 'undefined__esv6__')
if 'undefined__esv6__' in __symbols__:
    import module_collections
# __pragma__ ('endif')

import module_datetime

# __pragma__ ('ifdef', '__esv6__')
if '__esv6__' in __symbols__:
    import module_itertools
# __pragma__ ('endif')

import module_math
import modules
import nonlocals
import operator_overloading
import properties
import reprtest

# __pragma__ ('ifdef', '__esv6__')
if '__esv6__' in __symbols__:
    import proxies
# __pragma__ ('endif')

import set_comprehensions
import simple_and_augmented_assignment

#__pragma__ ('ifdef', '__sform__')
if '__sform__' in __symbols__:
    import string_format
#__pragma__ ('endif')

import truthyness
import tuple_assignment

autoTester = org.transcrypt.autotester.AutoTester ()

autoTester.run (arguments, 'arguments')
autoTester.run (attribs_by_name, 'attribs_by_name')
autoTester.run (builtin_super, 'builtin_super')
autoTester.run (byte_arrays, 'byte_arrays')
autoTester.run (callable_test, 'callable')
autoTester.run (classes, 'classes')
autoTester.run (complex_numbers, 'complex_numbers')
autoTester.run (conditional_expressions, 'conditional_expressions')
autoTester.run (control_structures, 'control_structures')

# __pragma__ ('ifdef', '__py3.6__')
if '__py3.6__' in __symbols__:
    autoTester.run (dashed_numbers, 'dashed_numbers')
# __pragma__ ('endif')

autoTester.run (data_structures, 'data_structures')
autoTester.run (decorators, 'decorators')
autoTester.run (dict_comprehensions, 'dict_comprehensions')
autoTester.run (dictionaries, 'dictionaries')
autoTester.run (div_issues, 'div_issues')
autoTester.run (div_pulls, 'div_pulls')
autoTester.run (docstrings, 'docstrings')
autoTester.run (exceptions, 'exceptions')
autoTester.run (extended_slices, 'extended_slices')

# __pragma__ ('ifdef', '__py3.6__')
if '__py3.6__' in __symbols__:
    autoTester.run (fstrings, 'fstrings')
# __pragma__ ('endif')

autoTester.run (globals_function, 'globals_function')
autoTester.run (general_functions, 'general_functions')
autoTester.run (indices_and_slices, 'indices_and_slices')

# __pragma__ ('ifdef', '__esv6__')
if '__esv6__' in __symbols__:
    autoTester.run (iterators_and_generators, 'iterators_and_generators')
# __pragma__ ('endif')
autoTester.run (lambda_functions, 'lambda_functions')
autoTester.run (list_comprehensions, 'list_comprehensions')
autoTester.run (local_classes, 'local_classes')
autoTester.run (metaclasses, 'metaclasses')
autoTester.run (method_and_class_decorators, 'method_and_class_decorators')
autoTester.run (module_builtin, 'module_builtin')
autoTester.run (module_cmath, 'module_cmath')

# __pragma__ ('ifdef', 'undefined__esv6__')
if 'undefined__esv6__' in __symbols__:
    autoTester.run (module_collections, 'module_collections')
# __pragma__ ('endif')

autoTester.run (module_datetime, 'module_datetime')

# __pragma__ ('ifdef', '__esv6__')
if '__esv6__' in __symbols__:
    autoTester.run (module_itertools, 'module_itertools')
# __pragma__ ('endif')
autoTester.run (module_math, 'module_math')
autoTester.run (modules, 'modules')
autoTester.run (nonlocals, 'nonlocals')
autoTester.run (operator_overloading, 'operator_overloading')
autoTester.run (properties, 'properties')
autoTester.run (reprtest, 'repr_str')

# __pragma__ ('ifdef', '__esv6__')
if '__esv6__' in __symbols__:
    autoTester.run (proxies, 'proxies')
# __pragma__ ('endif')

autoTester.run (set_comprehensions, 'set_comprehensions')
autoTester.run (simple_and_augmented_assignment, 'simple_and_augmented_assignment')

#__pragma__ ('ifdef', '__sform__')
if '__sform__' in __symbols__:
    autoTester.run (string_format, 'string_format')
#__pragma__ ('endif')
autoTester.run (truthyness, 'truthyness')
autoTester.run (tuple_assignment, 'tuple_assignment')

autoTester.done ()

Note that in the example above, the compilation command is transcrypt -e 6 autotest.py. The -e 6 switch forces compilation in JavaScript 6 mode and automatically adds e6 to the global list of defined symbols. The presence of the e6 symbol activates compilation of Transcrypt code that needs JavaScript 6 mode, by use of the __pragma__ ('ifdef', 'e6') and __pragma__ ('endif').

Code after __pragma__ ('ifndef', <symbol>) is compiled if <symbol> is NOT defined. To choose between two alternative source code blocks, precede the second block with __pragma__ ('else') and terminate it with __pragma__ ('endif').

Important: The conditional compilation pragma's also work when placed at the start of a line in precompiled or hand-written JavaScript code.

3.12. Inserting literal JavaScript: __pragma__ ('js', ...) and __include__ (...)

During compilation the __pragma__ ('js', code, <format parameters>) is replaced by the JavaScript code given in the code parameter. This code is formatted using the Python str.format method, using <format parameters>.

An example of its use is to encapsulate a JavaScript library as a Python module, as is shown for the fabric.js library. In that case there's usually one format parameter, namely a call to __include__ (<relative module path>). The module path is either relative to the directory holding the main module of your project, or to the root of the modules directory, and searched in that order. So modules local to your project prevail over generally available modules.

Note that since {} is used as placeholder in Python formatting, any normal { and } in your JavaScript in the code parameter have to be doubled. If you just want to include a literal piece of JavaScript without any replacements, you can avoid this doubling by using __pragma__ ('js', '{}', <my_piece_of_JS_code>).

3.13. Using an external transpiler: __pragma__ ('xtrans', <translator>, ..., cwd = <workingdirectory>)

This pragma works the same as __pragma__ ('js', ...), only now an external translator application can be specified, e.g. to translate JSX code. If the cwd parameter is present, it the external translator will use it as working directory. The code offered to this pragma is routed through this external translator by means of pipes. This pragma can use __include__ (...) just like __pragma__ ('js', ...).


Test program using __pragma__ ('xtrans', ...)
a = 1

__pragma__ ('xtrans', 'change_case.exe', '{}',
<div id="StudentContainer">
    {self.props.students.map((s) => (
        <StudentTile student={s}  key={s.searchval}  />
    cwd = 'workdir', 

__pragma__ ('xtrans', 'change_case.exe -l', '{}',
<div id="StudentContainer">
    {self.props.students.map((s) => (
        <StudentTile student={s}  key={s.searchval}  />
    cwd = 'workdir', 

b = 2
Simple external translator change_case.cpp, changing the case to upper or lower, depending on -l switch
#include <iostream>
#include <string>
#include <algorithm>
#include <fstream>

using namespace std;

int main (int argc, char *argv []) {
    string buffer;
    getline (cin, buffer, '\f');
    if (argc > 1 && string (argv [1]) == "-l") {
        transform (buffer.begin(), buffer.end(), buffer.begin(), ::tolower);
    else {
        transform (buffer.begin(), buffer.end(), buffer.begin(), ::toupper);
    cout << buffer;
    // Check if cwd parameter works correctly
    ofstream outputFile ("output.txt");
    outputFile << buffer;
    outputFile.close ();
The resulting translated file test.mod.js
    (function () {
        var a = 1;
            {SELF.PROPS.STUDENTS.MAP((S) => (
        <div id="studentcontainer">
            {self.props.students.map((s) => (
                <studenttile student={s}  key={s.searchval}  />
        var b = 2;
        __pragma__ ('<all>')
            __all__.a = a;
            __all__.b = b;
        __pragma__ ('</all>')
    }) ();

3.14. Create bare JavaScript objects and iterate over their attributes from Python: __pragma__ ('jsiter') and __pragma__ ('nojsiter')

Normally a Python {...} literal is compiled to dict ({...}) to include the special attributes and methods of a Python dict, including e.g. an iterator. When __pragma__ ('jsiter') is active, a Python {...} literal is compiled to a bare {...}, without special attributes or methods. To still be able to iterate over the attributes of such a bare JavaScript object from Python, when __pragma__ ('jsiter') is active, a Python for ... in ... is literally translated to a JavaScript for (var ... in ...). The main use case for this pragma is conveniently looping through class attributes in the __new__ method of a metaclass. As a more flexible, but less convenient alternative, __pragma__ ('js', '{}', '''...''') can be used.

An example of the use of this pragma is the following:

Use of __pragma__ ('jsiter') and __pragma ('nojsiter') to manipulate class attributes in a metaclass

class UppercaserMeta (type):
    def __new__ (meta, name, bases, attribs):
        __pragma__ ('jsiter')       # Translate for ... in directly to JavaScript for ... in ... and translate {} to bare {} rather than to dict {}
                                    # Using bare {} as attribs parameter to __new__ avoids dict attributes masking regular attributes
                                    # For more flexibility use __pragma__ ('js', '{}', '''...''')
        upperAttribs = {}
        for attribKey in attribs:   # Translates to 'for (var attribKey in attribs)' by virtue of __pragma__ ('jsiter'), to iterate over the attributes of a bare JavaScript {}
            upperAttribs [attribKey if  attribKey.startswith ('__') else attribKey.upper ()] = attribs [attribKey]
        __pragma__ ('nojsiter')
        return type.__new__ (meta, name, bases, upperAttribs)

3.15. __pragma__ ('jskeys') and __pragma__ ('nojskeys')

Normally in Python, dictionary keys without quotes are interpreted as identifiers and dictionary keys with quotes as string literals. This is more flexible than the JavaScript approach, where dictionary keys with or without quotes are always interpreted as string literals. However to better match the documentation and habits with some JavaScript libraries, such as plotly.js, dictionary keys without quotes can be optionally interpreted as string literals. While the -jk command line switch achieves this globally, the preferred way is to switch on and off this facility locally.

3.16. __pragma__ ('keycheck') and __pragma__ ('nokeycheck')

When __pragma__ ('keycheck') is active, an attempt to retrieve an element from a dictionary via a non-existing key results in a KeyError being raised, unless it happens at the lefthand side of an augmented assignment. The run time support for this is expensive, as it requires passing the result of every such retrieval to a function that checks the definedness of the result. While the -kc command line switch switches key checking on globally, the preferred way is to switch on and off this facility locally to prevent code bloat.

3.17. Keeping your code lean: __pragma__ ('jsmod') and __pragma__ ('nojsmod')

When __pragma__ ('jsmod') is active, % has JavaScript rather than Python semantics. While the -jm command line switch achieves this globally, the preferred way is to switch on and off this facility locally.

3.18. Keeping your code lean: __pragma__ ('kwargs') and __pragma__ ('nokwargs')

While it's possible to compile with the -k command line switch, allowing keyword arguments in all flavors supported by Python 3.5 in all places, this disadvised, as it leads to bloated code. It is better to use the 'kwargs' and 'nokwargs' pragmas, to enable this feature only at definition (as opposed to calling) of functions that require it. You'll find an example of how to use these pragma's in the arguments autotest. You can use them on whole modules or any part thereof. Note that at due to the dynamic nature of Python, use of keyword arguments at call time cannot be predicted at definition time. When running with CPython from the command prompt using the browser stubs, these pragma's are ignored.

3.19. Preventing target annotation: __pragma__ ('noanno')

The -a command line switch will comment target code compiled from Python with source file names and source line numbers. Since this interferes with literal inclusion of JavaScript code with multi-line comments, it can be switched off by including __pragma__ ('noanno') at the beginning of a module. An example of this can be seen in the encapsulation code for the fabric.js library.

3.20. Operator overloading: __pragma__ ('opov') and __pragma__ ('noopov')

Transcrypt currently supports overloading of the following operators: *, /, +, -, @, [], (), ==, !=, <, <=, >, and >=.

These operators have been chosen since they can enhance the readability of computations involving matrices and vectors, enable the use of callable objects (functors) or are used to compare tuples, lists and sets. Using the -o command line switch will activate operator overloading globally. This is strongly disadvised, since even 1 + 2 will result in two function calls in that case. It's better to use __pragma__ ('opov') to switch it on locally and __pragma__ ('noopov') to switch it off again, activating operator overloading only for lines of code involving extensive matrix / vector computations or functor calls (as opposed to definitions), in which case the overhead is negligeable in most cases.

Formula v4 = M * (v1 + v2) + v3 is probably preferred over v4 = add (mul (M, add (v1, v2), v3)), which by the way closely resembles the JavaScript that will be generated for the expression that uses overloaded + and * operators. In order to support operator overloading your matrix and vector classes have to support the appropriate selection of functions out of __mul__, __rmul__, __div__, __rdiv__, __add__, __radd__, __sub__, __rsub__, __matmul__, __rmatmul__, __getitem__, __setitem__ and __call__ as described in the CPython documentation. The comparison operators are supported by the functions __eq__, __ne__, __lt__, __le__, __gt__ and __ge__.

3.21. Skipping fragments while generating code : __pragma__ ('skip') and __pragma__ ('noskip')

On some occasions it's handy to be able to skip code generation for fragments of Python source. The most important use case is when you perform a static check using the -c or --check command line switch. If you use global JavaScript names without importing them from some encapsulation module, you'll have to tell the static checker that these names are not to be flagged as undefined. Insert a line containing dummy definitions of the unknown names for the static checker, surrounded by __pragma__ ('skip') and __pragma__ ('noskip'), to make code generator skip these definitions. The beginning of the source code of the Pong demo, shown below, demonstates how this is done.

Preventing complaints of the static checker about unknown global JavaScript names in pong.py
__pragma__ ('skip')
document = window = Math = Date = 0 # Prevent complaints by optional static checker
__pragma__ ('noskip')

__pragma__ ('noalias', 'clear')

from com.fabricjs import fabric

orthoWidth = 1000
orthoHeight = 750
fieldHeight = 650

enter, esc, space = 13, 27, 32

window.onkeydown = lambda event: event.keyCode != space # Prevent scrolldown on spacebar press

3.22. Automatic conversion to truth value: __pragma__ ('tconv') and __pragma__ ('notconv')

When automatic conversion to truth value is activated, an empty array, dict or set has truth value False just as in CPyton, rather than True, as would be the case natively in JavaScript. This comes at the expense of a runtime typecheck and is by default avoided in Transcrypt for that reason. To switch on automatic conversion to truth values locally, __pragma__ ('tconv') and __pragma__ ('notconv') can be used. The alternative is to switch on automatic conversion globally using the -t command line switch. Use of this switch is disadvised if a lot of boolean expressions are computed in inner loops, since it would result in many extra typechecks. When designing performance sensitive libraries, it's advisable to use __pragma__ ('notconv') explicitly at the start of each performance-sensitive module. The result will be that even when an application developer chooses to use the -t switch, performance of won't suffer.

3.23. Adding directories to the module search path: __pragma__ ('xpath', <directory list>)

In addition to the regular module mechanism you can use # __pragma__ ('xpath', <directory list>) to add directories to the module search path. Note that resolving search paths happens compile time, so dynamic manipulation of the module search path doesn't apply. If you want to alter the search path in code that is used both by Transcrypt and CPython, you can additionally manipulate sys.path in a try clause or under an if with condition __envir__.executor_name == __envir__.interpreter_name.

The -xp / --xpath command line switch has the same effect.