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Code Generation in Python Dismantling Jinja a

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bit.ly/codegeneration Discuss this presentation, give feedback

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Code Generation?

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eval is evil Or is it?

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Why is eval evil? Security Performance &

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Security Code Injection Namespace pollution

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Performance No bytecode Code makes code that code runs

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So: Why? No suitable alternatives

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use responsibly because of this:

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101 EVAl

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>>> code = compile('a = 1 + 2', '', 'exec') >>> code at 0x1004d5120, file "", line 1> Compile

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>>> ns = {} >>> exec code in ns >>> ns['a'] 3 Eval

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>>> import ast >>> ast.parse('a = 1 + 2') <_ast.Module object at 0x1004fd250> >>> code = compile(_, '', 'exec') AST #1

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AST #2 >>> n = ast.Module([ ... ast.Assign([ast.Name('a', ast.Store())], ... ast.BinOp(ast.Num(1), ast.Add(), ... ast.Num(2)))])) >>> ast.fix_missing_locations(n) >>> code = compile(n, '', 'exec')

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No strings passed to eval()/exec Explicit compilation to bytecode Execution in explicit namespace Recap

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ARCHITECTURE TeMpLAtE eNgInE

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No content

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2nd Iteration Generates Python Code Python Semantics Different Scoping Overview

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Lexer Pipeline Parser Identifier Analyzer Code Generator Python Source Bytecode Runtime

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Different Scoping WSGI & Generating Debug-ability Restricted Environments Complexities

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    {% for item in seq %}
  • {{ item }}
    • {% endfor %}
Input

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print "
    " for each item in the variable seq push the scope print "
  • " print the value of item and escape it as necessary print "
    • " pop the scope print "
" Behavior

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Naive: write(u'
    ') for _tmp in context['seq']: context.push({'item': _tmp}) write(u'
  • ') write(autoescape(context['item'])) write(u'
    • ') context.pop() write(u'
')

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Actual: l_seq = context.resolve('seq') write(u'
    ') for l_item in l_seq: write(u'
  • ') write(autoescape(l_item)) write(u'
    • ') write(u'
')

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?

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COMPILATION INtRoDUCTIoN tO

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Low Level High Level e Art of Code Generation versus

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Low Level Code Generation 2 0 LOAD_CONST 1 (1) 3 LOAD_CONST 2 (2) 6 BINARY_ADD 7 STORE_FAST 0 (a) a = 1 + 2

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Assign(targets=[Name(id='a', ctx=Store())], value=BinOp(left=Num(n=1), op=Add(), right=Num(n=2)))] a = 1 + 2 High Level Code Generation

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Bytecode Abstract Syntax Trees Sourcecode Building Blocks

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Bytecode Undocumented Does not work on GAE Implementation Specific

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AST More Limited Easier to Debug Does not segfault the Interpreter

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Source Works always Very Limited Hard to Debug without Hacks

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e Tale of Two Pieces of Code (very similar pieces of code)

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def foo(): a = 0 for x in xrange(100): a += x print a foo() Fast

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a = 0 for x in xrange(100): a += x print a Slower

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?

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2 0 LOAD_CONST 0 (0) 3 STORE_NAME 0 (a) 3 6 SETUP_LOOP 30 (to 39) 9 LOAD_NAME 1 (xrange) 12 LOAD_CONST 1 (100) 15 CALL_FUNCTION 1 18 GET_ITER >> 19 FOR_ITER 16 (to 38) 22 STORE_NAME 2 (x) 4 25 LOAD_NAME 0 (a) 28 LOAD_NAME 2 (x) 31 INPLACE_ADD 32 STORE_NAME 0 (a) 35 JUMP_ABSOLUTE 19 >> 38 POP_BLOCK 5 >> 39 LOAD_NAME 0 (a) 42 PRINT_ITEM 43 PRINT_NEWLINE Slower

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2 0 LOAD_CONST 1 (0) 3 STORE_FAST 0 (a) 3 6 SETUP_LOOP 30 (to 39) 9 LOAD_GLOBAL 0 (xrange) 12 LOAD_CONST 2 (100) 15 CALL_FUNCTION 1 18 GET_ITER >> 19 FOR_ITER 16 (to 38) 22 STORE_FAST 1 (x) 4 25 LOAD_FAST 0 (a) 28 LOAD_FAST 1 (x) 31 INPLACE_ADD 32 STORE_FAST 0 (a) 35 JUMP_ABSOLUTE 19 >> 38 POP_BLOCK 5 >> 39 LOAD_FAST 0 (a) 42 PRINT_ITEM 43 PRINT_NEWLINE Fast

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2 0 LOAD_CONST 1 (0) 3 STORE_FAST 0 (a) 3 6 SETUP_LOOP 30 (to 39) 9 LOAD_GLOBAL 0 (xrange) 12 LOAD_CONST 2 (100) 15 CALL_FUNCTION 1 18 GET_ITER >> 19 FOR_ITER 16 (to 38) 22 STORE_FAST 1 (x) 4 25 LOAD_FAST 0 (a) 28 LOAD_FAST 1 (x) 31 INPLACE_ADD 32 STORE_FAST 0 (a) 35 JUMP_ABSOLUTE 19 >> 38 POP_BLOCK 5 >> 39 LOAD_FAST 0 (a) 42 PRINT_ITEM 43 PRINT_NEWLINE Fast

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>>> def foo(): ... a = 42 ... locals()['a'] = 23 ... return a ... >>> foo() 42 Example

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SEMANTICS A StOrY ABoUT

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print "
    " for each item in the variable seq push the scope print "
  • " print the value of item and escape it as necessary print "
    • " pop the scope print "
" Remember

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at's not how Python works … so how do you generate code for it?

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Keep tracks of identifiers emulate desired semantics Tracking

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Context in Jinja2 is a Data Source Context in Django is a Data Store Scopes

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    {% for item in seq %} {% include "item.html" %} {% endfor %}
Source

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Code l_seq = context.resolve('seq') write(u'
    ') for l_item in l_seq: t1 = env.get_template('other.html') for event in yield_from(t1, context, {'item': l_item}) yield event write(u'
')

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What happens in the include … … stays in the include

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Impossible @contextfunction def get_users_and_store(context, var='users'): context[var] = get_all_users() return u''

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EXAMPLES PrACTICAl

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Source
    {% for item in sequence %}
  • {{ item }}
    • {% endfor %}

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Generated def root(context): l_sequence = context.resolve('sequence') yield u'\n
    \n' l_item = missing for l_item in l_sequence: yield u'\n
  • %s
    • ' % ( escape(l_item), ) l_item = missing yield u'\n
'

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Source
    {% for item in sequence %}
  • {{ loop.index }}: {{ item }}
    • {% endfor %}

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Generated def root(context): l_sequence = context.resolve('sequence') yield u'\n
    \n' l_item = missing for l_item, l_loop in LoopContext(l_sequence): yield u'\n
  • %s: %s
    • \n' % ( escape(environment.getattr(l_loop, 'index')), escape(l_item), ) l_item = missing yield u'\n
'

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Source
    {% for item in sequence %}
  • {{ loop.index }}: {{ item }}
    • {% endfor %}

Item: {{ item }}

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Generated def root(context): l_item = context.resolve('item') l_sequence = context.resolve('sequence') yield u'\n
    \n' t_1 = l_item for l_item, l_loop in LoopContext(l_sequence): yield u'\n
  • %s: %s
    • \n' % ( escape(environment.getattr(l_loop, 'index')), escape(l_item), ) l_item = t_1 yield u'\n
\n

Item: ' yield escape(l_item)

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Source {% extends "layout.html" %} {% block body %}

Hello World!

{% endblock %}

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Generated def root(context): parent_template = environment.get_template('layout.html', None) for name, parent_block in parent_template.blocks.iteritems(): context.blocks.setdefault(name, []).append(parent_block) for event in parent_template.root_render_func(context): yield event def block_body(context): if 0: yield None yield u'\n

Hello World!

\n' blocks = {'body': block_body}

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Source {% block body %}{% endblock %}

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Generated def root(context): yield u'\n' for event in context.blocks['body'][0](context): yield event def block_body(context): if 0: yield None blocks = {'body': block_body}

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Source {% extends "layout.html" %} {% block title %}Hello | {{ super() }}{% endblock %}

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Generated def root(context): parent_template = environment.get_template('layout.html', None) for name, parent_block in parent_template.blocks.iteritems(): context.blocks.setdefault(name, []).append(parent_block) for event in parent_template.root_render_func(context): yield event def block_title(context): l_super = context.super('title', block_title) yield u'Hello | ' yield escape(context.call(l_super)) blocks = {'title': block_title}

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Jinja Do WHY DOeS

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… manual code generation? why Originally the only option AST compilation was new in 2.6 GAE traditionally did not allow it

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… generators instead of buffer.append() why Required for WSGI streaming unless greenlets are in use Downside: StopIteration :-(

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… map "var_x" to "l_var_x" why Reversible to debugging purposes Does not clash with internals see templatetk for better approach

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Jinja Do HOW DoEs

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… does automatic escaping work how Markup object Operator overloading Compile-time and Runtime

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Const

{{ "Hello World!" }}

def root(context): yield u'

<strong>Hello World!</strong>

'

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Runtime

{{ variable }}

def root(context): l_variable = context.resolve('variable') yield u'

%s

' % ( escape(l_variable), )

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Control #1 {% autoescape false %}

{{ variable }}

{% endautoescape %} def root(context): l_variable = context.resolve('variable') t_1 = context.eval_ctx.save() context.eval_ctx.autoescape = False yield u'

%s

' % ( l_variable, ) context.eval_ctx.revert(t_1)

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Control #2 {% autoescape flag %}

{{ variable }}

{% endautoescape %} def root(context): l_variable = context.resolve('variable') l_flag = context.resolve('flag') t_1 = context.eval_ctx.save() context.eval_ctx.autoescape = l_flag yield u'%s%s%s' % ( (context.eval_ctx.autoescape and escape or to_string)((context.eval_ctx.autoescape and Markup or identity)(u'

')), (context.eval_ctx.autoescape and escape or to_string)(l_variable), (context.eval_ctx.autoescape and escape or to_string)((context.eval_ctx.autoescape and Markup or identity)(u'

')), ) context.eval_ctx.revert(t_1)

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… far does the Markup object go? how All operators are overloaded All string operations are safe necessary due to operator support

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Example >>> from markupsafe import Markup >>> Markup('%s') % '' Markup(u'<insecure>') >>> Markup('') + '' + Markup('') Markup(u'<insecure>') >>> Markup('Complex value').striptags() u'Complex\xa0value'

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… do unde ned values work how Configurable Replaced by special object By default one level of silence

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Example >>> from jinja2 import Undefined >>> unicode(Undefined(name='missing_var')) u'' >>> unicode(Undefined(name='missing_var').attribute) Traceback (most recent call last): File "", line 1, in UndefinedError: 'missing_var' is undefined

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Q&A

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@mitsuhiko http://lucumr.pocoo.org/ armin.ronacher@active-4.com

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Oh hai. We're hiring http://fireteam.net/careers