pymacros4py

Brief Description

What is pymacros4py?

pymacros4py is a Python templating system designed as a source-level macro preprocessor. It allows you to create and embed macros (written in Python) that manipulate, duplicate, or generate Python code. The system helps ensure that the generated code meets Python's indentation requirements.

Why Use pymacros4py?

While macros are mostly unnecessary in Python due to its rich syntax and dynamic capabilities, there are specific scenarios where using a macro-based templating system can be very effective. Examples include:

• Cross-platform requirements: Maintaining code for multiple platforms (e.g., CPython and Cython) where certain sections of code need to be specific to each platform.
• Performance-critical code: Inlining functions in performance-sensitive areas without duplicating their implementation all over the project or making significant changes like moving to PyPy.
• Code consistency: Simplifying the management of repetitive code structures (e.g., function signatures or identical logic) to ensure consistency.
• Configuration automation: Explicitly generating portions of configuration files instead of relying on hidden generator scripts.

If you're seeking for a solution to address such challenges, pymacros4py could be helpful for you.

(Note: pymacros4py expands macros at the source level. For extending the language Python itself, in order to get a programming language with additional capabilities, macro expansion on the level of the abstract syntax tree, in the build phase of the interpretation process, is better suited. See, e.g., the language lab mcpyrate.)

Key Features

• Designed specifically for Python file templates: Macro-generated code adheres to Python's indentation rules seamlessly.
• Macro domain elements such as statements, functions, and expressions are defined directly in Python code: No need to learn a specialized macro language - you simply embed Python macro code within Python code. This approach allows you to leverage all the functionalities Python offers.
• Operates at the source level: Using a preprocessor provides visibility into the generated code prior to bytecode generation, compilation, or interpretation. Additionally, it avoids introducing dependencies on external code, libraries, or executables during these phases.
• Macro variables and functions: These can be declared either directly in the files they are utilized in or stored in separate files for better organization.
• Supports multi-level macro expansion: For example, macro code can include templates containing additional macro code. Expansion results are cached to eliminate redundant computations and boost efficiency.
• Strictly limits replacements to explicitly marked macro sections: This prevents unintended consequences where a macro definition could inadvertently apply to future code additions by others, ensuring predictable and manageable outcomes.

pymacros4py is implemented as pure Python code, with a small code base. It could be an acceptable pre-build dependency for your project, even if you aim at avoiding dependencies. And if no dependency is acceptable, just copy its code directly into your project.

The code of pymacros4py is tested with 100% code coverage. Additionally, every example in this README - except for a single line (excluded from the tests for technical constraints) - is covered by tests.

At present, semantic versioning is not applied. The practical experience available with the library so far is not sufficient to assess the stability of the API.

First Examples

The following simple examples demonstrate how templating can be used.

Example: Signatures, docstrings and code kept consistent by templating

The following template combines macro code with application code. The macro code is the Python code enclosed between the $$ symbols within the quotes.

'''$$
    def signature_fragment():
        insert(f"increment: bool = True\n")

    def code_fragment(v):
        insert(f'''\
"""If *increment* is True, return one more than *{v}*,
and otherwise, one less than *{v}*.
"""
return {v + 1} if increment else {v - 1}
''')

$$'''
def increment_or_decrement_1(
    # $$ signature_fragment()
) -> int:
    # $$ code_fragment(1)

def increment_or_decrement_2(
    # $$ signature_fragment()
) -> int:
    # $$ code_fragment(2)

This template produces the following result:

def increment_or_decrement_1(
    increment: bool = True
) -> int:
    """If *increment* is True, return one more than *1*,
    and otherwise, one less than *1*.
    """
    return 2 if increment else 0

def increment_or_decrement_2(
    increment: bool = True
) -> int:
    """If *increment* is True, return one more than *2*,
    and otherwise, one less than *2*.
    """
    return 3 if increment else 1

Like this, macro code with its definitions and usages can simply be written in Python.

Additionally, it's possible to use fragments of the template as body of function definitions, even at the point where these functions are first used. For instance, the template below produces the same result as the one above:

def increment_or_decrement_1(
    # $$ def signature_fragment():
    increment: bool = True
    # $$ :end
    # $$ signature_fragment()
) -> int:
    # $$ def code_fragment(v):
    """If *increment* is True, return one more than *'$$ insert(v) $$'*,
    and otherwise, one less than *'$$ insert(v) $$'*.
    """
    return '$$ insert(v+1) $$' if increment else '$$ insert(v-1) $$'
    # $$ :end
    # $$ code_fragment(1)

def increment_or_decrement_2(
    # $$ signature_fragment()
) -> int:
    # $$ code_fragment(2)

Example: Current year inserted into license file

While many features of pymacros4py are designed specifically for templating Python files - such as proper handling of indentation in both macros and application code - templates can also be used for other text files within your project.

For example, the following template demonstrates how to automatically insert the current year into a license file:

'''$$
import time
def current_year():
    now = time.gmtime()
    return time.strftime('%Y', now)
$$'''
MIT License
Copyright(c) 2023 - '$$ insert(current_year()) $$' The author
...

This template generates:

MIT License
Copyright(c) 2023 - 2025 The author
...

Example: Pre-computation of expressions

The following template defines an expression as a macro function and executes it during the macro expansion, in the sense of a pre-computation.

'''$$
   def incremented_by_1(i):
       insert(i+1)
$$'''

print('$$ incremented_by_1(5) $$')
print('$$ incremented_by_1(6) $$')

This template generates:

print(6)
print(7)

Example: In-lining of expressions and function statements

The following template defines an expression as a macro function and inserts it during the macro expansion, in the sense of an in-lining.

'''$$
   def increment_by_1(i):
       insert(f'({i} + 1)')
$$'''

print('$$ increment_by_1("3") $$' * 2)

print((3 + 1) * 2)

Usage

Installation

You can get and install pymacros4py as follows by using pip:

pip install pymacros4py

Calling pymacros4py

The preprocessor can be accessed in a Python script as follows:

>>> import pymacros4py
>>> pp = pymacros4py.PreProcessor()

Then, the macros in a template file can be expanded as follows:

pp.expand_file_to_file('my_code.template.py', 'my_code.py')

Warning: When pymacros4py expands macros, it executes the expressions, statements and functions of your macro code. You might want to apply it only to files that you have fully under your control. If you write macro code that includes unsafe file content, you can disable macro expansion for this content.

The method expand_file_to_file offers the following keyword parameter:

• diffs_to_result_file: bool = False. True means that the result is not written to the output file, but compared to the content of the file, and the result of the comparison is returned.

>>> pp.expand_file_to_file('tpl/README.rst', 'README.rst', diffs_to_result_file=True)
''

If you need specific arguments for the parameters encoding, errors, or newlines used for opening files (see Python function open), you can set these as attributes of the global object file_options:

>>> pymacros4py.file_options.encoding = "utf-8"

Using pymacros4py With a Code Formatter

If you wish to utilize a code formatter like Black on files generated through template expansion, pymacros4py offers its functionality at a slightly lower level of abstraction to support such implementations. This is showcased in the following example. It uses all the functionality provided.

Just expand a template, do not write to a file:

>>> template_path = 'tests/data/file_generating_unformatted_code.tpl.py'
>>> expanded = pp.expand_file(template_path)

Process the results of a macro expansion with an external tool:

>>> temp_file_path = pymacros4py.write_to_tempfile(expanded)
>>> pymacros4py.run_process_with_file(["black", temp_file_path], temp_file_path)
>>> formatted = pymacros4py.read_file(temp_file_path, finally_remove=True)

Store the results of the post-processing to a file:

>>> result_path = 'tests/data/file_generating_unformatted_code.py'
>>> pymacros4py.write_file(result_path, formatted)

Compare two texts, e.g., a template and the expanded result, or an expanded result and a formatted form of its content, in the form used by option diffs_to_result_file of method pp.expand_file_to_file:

>>> print(pp.diff(expanded, formatted, "expanded", "formatted"))
*** expanded
--- formatted
***************
*** 1,3 ****
! print(
! "Hello world"
! )
--- 1 ----
! print("Hello world")
<BLANKLINE>

Notes:

• read_file, expand_file, write_file, and write_to_tempfile all use the file_options described above.
• run_process_with_file calls subprocess.run. The first argument is used as args for this function of the Python standard lib. See there for details. The second argument gives the path of the processed file to pymacros4py, so that the library can hint to the file in case an exception is raised during the run.
• If run_process_with_file raises an exception, the temporary file is not removed: The user might want to examine the content of the file the external code formatter got as input, because the reason of the failure could be within the processed file.

Templates and Template Expansion

A template consists of macro sections and text sections. A single line of a template can already contain several such sections.

• A macro section contains Python code intended to be executed during the macro expansion.
• A text section can be anything. In case of a template for a Python file, it is normal Python code. It is used as-is (except for a possible adaptation of the indentation).

For expanding the macros in a template, pymacros4py separates the macro and the text sections. Then, it generates and executes a so-called template script as follows:

• Code of macro sections of the template is directly taken into the template script. When this code is executed, it can insert text into the output of the macro expansion by calling function insert().
• For text sections, a statement that inserts the text into the results is automatically appended to the template script.

Example: The following template for application code contains a full-line macro section (the first line) and a macro section embedded in a line of the application code

# $$ v = 2 * 3
x = '$$ insert(v) $$'

From this template, pymacros4py generates a template script that looks roughly as follows:

v = 2 * 3
insert('x = ')
insert(v)
insert('\n')

This template script will be executed by pymacros4py. It generates the following application code as result:

x = 6

Application code written in Python and macro code written in Python can be mixed like this, and the macro code extends and manipulates the application code.

This explanation and example already gives a good impression of how templates can be written. Further details are described in the following sections.

Quoted Macro Code in Templates

One way to mark macro code in a template looks similar to a string starting and ending with two dollar characters. Single or double quotes, or triple single or double quotes can be used.

Note, that whitespace between the quotes and the dollar characters is not allowed.

Example: The following lines each show a macro section with 'v = 0' as macro code within the macro section.

'$$ v = 0 $$'
"$$ v = 0 $$"
'''$$ v = 0 $$'''
"""$$ v = 0 $$"""

Start and end of macro code is identified only by the special combination of quoting and dollar characters. Thus, both the quotes and the dollars can be freely used in macro code and in application code, as long as they do not occur directly together. This makes the macro recognition quite robust.

Example: Some dollar characters and quotes in application code and in macro sections, but not combined in the special syntax that starts or ends a macro section

print('This is application code with $$ characters and quotes')
'$$ v = 'This is a quoted string within macro code' $$'

A macro section spans quoting, dollars and code together.

If before and after the quotes, there are only space or tab characters, the macro section is a block macro section (otherwise: an inline macro section) and spans the whole line(s), including a trailing line break if present.

Example: Macro section that spans the whole line, including the trailing line break.

# This is a comment in application code
'$$ v = 0  # This macro section spans the whole line $$'
# This is a second comment in application code

Macro code can span several lines. All four possible quoting types can be used for this, but triple quotes are more pythonic here.

Example: Macro section that spans several lines

'''$$ # This comment belongs to the macro code
      v = 'a string'
$$'''

Whitespace and line breaks in the macro section before and after the macro code are ignored.

Example: Identical macro code ('v = 0'), surrounded by different whitespace and/or line breaks.

'''$$      v = 0       $$'''
'''$$
      v = 0
$$'''

Indentation in Macro Code

Macro code in a macro section can be indented to an arbitrary local level, independently of other macro sections and surrounding application code. Locally, indentation needs to follow Python syntax. Globally, pymacros4py will establish a valid indentation when combining code of several macro sections, and code generated by pymacros4py itself.

The first (non-whitespace) character of the macro code in a macro section defines the base indentation of the code. Subsequent lines of the macro code need to be indented accordingly: equally indented (by literally the same characters, but with each non-whitespace character replaced by a space character), or with additional indentation characters (following the base indentation), or not indented at all. When pymacros4py re-indents code, it changes only the base indentation, and it keeps non-indented lines non-indented.

Note: This concept supports indentation by space characters, by tabs, and even mixed forms, and does not require fixing the amount of indentation resulting from a tab. But there is one limitation: If macro code is indented by tabs, it needs to start in its own line.

Example: Macro code starts in its own line. Indentation is done by space and/or tab characters. The indentation of the first non-whitespace character (here: 'v') defines the base indentation of the macro section, and subsequent lines are indented equally (by an identical indentation string). The third line is locally indented, relative to this base indentation.

'''$$
        v1 = 0
        for i in range(3):
            v1 += i
$$'''

Example: Macro code starts in the first line of the macro section. All indentation is done by space characters. The number of characters left to the 'v' determines the base indentation that the second line follows. The third line is locally indented, relative to this base indentation.

'''$       v1 = 0
           for i in range(3):
               v1 += i
$$'''

Example: Multi-line string with non-indented content

'''$$
    v = """
First line of string. No indentation. This will be preserved.
Second line of string. No indentation. This will be preserved.
"""
    # Our code continues at the level of the base indentation
    insert(v)
$$'''

Macro Code within a Comment

A second option to mark macro code in a template has the form of a comment, starting with a hash character, optionally followed by spaces or tabs, and two dollar characters. The macro code ends with the line. If there are only space and/or tab characters before the hash, the macro section spans the whole line, including a trailing line break.

# $$ v = 0

Hint: Macro code in a comment is valid Python syntax even if it occurs within signatures in application code. This prevents a Python editor from signalling a syntax error in your template, what can be helpful. Quoted macro code has the advantage that it can span multiple lines, and that some editors highlight it in the template, what can also be useful.

Arbitrary Python Code as Macro Code

Macro code is regular Python code. A call to the predefined function insert inserts the results of applying the function str to the arguments of insert at the place of the macro section.

Example: Macro code defining a function that generates code

def a_function_of_our_application():
    '''$$
    # Here, we define a function in macro code
    def return_print_n_times(n, s):
        statement = f'print("{s}")\n'
        return statement * n
    # Now, we call it
    insert(return_print_n_times(3, "Yep."))
    $$'''

The template script derived from this template generates the following result:

def a_function_of_our_application():
    print("Yep.")
    print("Yep.")
    print("Yep.")

Example: Macro code inserting a computation result

def example_function(i: int) -> int:
    # $$ v = 2 * 2
    return '$$ insert(v) $$'

It evaluates to:

def example_function(i: int) -> int:
    return 4

Indentation of Macro Results

The results of the expansion of a macro section, e.g., the output of calls of function insert, are indented relative to the indentation of the first non-whitespace character of the macro section (i.e., the hash character for macro code in a comment, resp., the first quote in quoted macro code).

Example: Macro sections with different indentation levels

v = True
# $$ # Macro expansion result will be indented to this level
# $$ insert(f'print({1+1})\n')
if v:
    # $$ # Macro expansion result will be indended to this higher level
    # $$ insert(f'print({2+2})\n')

This template is evaluated to the following result:

v = True
print(2)
if v:
    print(4)

For inline macro sections, the first line of the results is inserted without adding indentation. For block macro sections, each line is (re-) indented.

Example: An inline macro section and a block macro section, both with multi-line results

# $$ v = 2
y = 1 + '$$ insert("(\n", v, "\n* w\n)") $$'
z = 11 + (
         '''$$ insert(v+1, "\n*w\n") $$'''
         )

This template is evaluated to the following result:

y = 1 + (
        2
        * w
        )
z = 11 + (
         3
         *w
         )

In the first case, the inline macro section, the expansion result (starting with the opening bracket) is inserted directly after the application code 'y = 1 + ', without indentation.

In the second case, the block macro section, the expansion result (starting with the '3') is inserted with indentation.

If the library detects zero indentation in macro output, this zero indentation is preserved, i.e., no re-indentation happens.

Example: Recognizable zero indentation in macro output is preserved.

if True:
    """$$
        insert("    v = '''\ntext\n'''\n")
    $$"""

This template is evaluated to the following result:

if True:
    v = '''
text
'''

The macro section of the example starts in an indented suite, here, of an if statement. Thus, macro output of the following macro code will be re-indented to this level - except for the case that zero indentation of output is explicitly demanded. So, we can check in the results, if this exception works.

Then, in the macro code, we start with inserting output at a non-zero base indentation, as reference (the spaces before the assignment). So, the library can detect that the subsequent lines require zero-indentation (the text of the string literal is given with zero indentation).

In the expansion result, we see that the macro output starts indented to the level of the start of the macro section: re-indentation happened here. But then, the zero indentation of the lines of the string literal is detected and thus preserved.

Including and Importing Files

Macro code can insert expansion results or import attributes, e.g., function definitions, from other template files. pymacros4py offers the following functions for this:

• insert_from(self, template_file: str, globals_dict: Optional[dict]=None) -> None:

  Perform a macro expansion of template_file within a new namespace, and insert the results into the results of the current macro expansion. globals can be given to initialize the namespace like in a call of eval().

  When called a second time with an identical argument for file, and globals is None in both calls, re-use the output of the previous run.

  (If globals is not None, and you want to re-use results in cases of equivalent content of globals, this has to be implemented manually.)

• import_from(self, template_file: str) -> None:

  Perform a macro expansion of template_file in the namespace of the current macro expansion (attributes that have already been set can be used by macro code in template_file, and attributes set by such code can be used in macro code following the call).

  Discard the output of the expansion run.

  When called a second time with an identical argument for template_file, ignore the call.

If the first part of the path (see pathlib.PurePath.parts) given as template_file is $$, this part is removed and the subsequent parts are interpreted relative to the directory of the currently expanded template.

Example for insert_from:

The following call of insert_from:

def example_function() -> int:
    # $$ i = 3
    # $$ insert_from("tests/data/file_with_output_macro.py")
    return "$$ insert(i) $$"

with the following content of the file:

# $$ i = 2
print('some text')

evaluates to:

def example_function() -> int:
    print('some text')
    return 3

The output of the include statement is added to the results, but the content of the global namespace (here: the value of variable i) is not changed.

Example for import_from, with a path relative to the template:

The following template:

# $$ import_from("$$/file_with_definition_macro.py")
# $$ insert(return_print_n_times(3, "Yep."))

with the following content of the file, that is stored in the same directory as the template that contains the above given code:

'''$$
    def return_print_n_times(n, s):
        statement = f'print("{s}")\n'
        return statement * n
$$'''
print("Text not important")

evaluates to:

print("Yep.")
print("Yep.")
print("Yep.")

The content of the global namespace is extended by function return_print_n_times, but the output of the imported template is ignored.

Macro Statement Suites Across Multiple Sections

If the code in a macro section ends within a suite of a Python compound statement (see https://docs.python.org/3/reference/compound_stmts.html) e.g., an indented block of statements after statements like if, for, or def, this suite ends with the macro code:

Example:

'''$$ v = 1
      if v == 0:
          insert("print('v == 0')")
$$'''
# $$ insert("print('Always')\n")

Result:

print('Always')

But a suite can also span over subsequent template or macro sections. This case is supported in a limited form (!) as follows:

• Start of the suite: Macro section with just the introducing statement

  The header of the compound statement (its introducing statement, ending with a colon) needs to be the only content of the macro section. Not even a comment is allowed after the colon.

  Reason: The beginning of a suite that is meant to span multiple sections is recognized by the colon ending the macro code. The kind of compound statement is recognized by the first word of the macro code.

• A suite is ended by a :end macro section

  If the code of a macro section just consists of the special statement :end, the suite that has started most recently, ends. Whitespace is ignored.

• Macro sections elif, else, except, finally, and case end a suite and start a new one

  If a macro section starts with one of the listed statements and ends with a colon, the suite ends, that has started most recently, the macro code is handled, and then a new suite starts.

• Such suites can be nested.

Examples for conditionally discarding or using text sections:

# $$ import datetime
# $$ d = datetime.date.today()
# $$ if d > datetime.date(2024, 1, 1):
# $$ code_block = 1
# This comes from the first macro code block, number '$$ insert(code_block) $$'
print('January 1st, 2024, or later')
# $$ else:
# This comes from the second macro code block, number '$$ insert(code_block) $$'
print('Earlier than January 1st, 2024')
# $$ :end

The template script generated from this template looks roughly as follows:

import datetime
d = datetime.date.today()
if d > datetime.date(2024, 1, 1):
    code_block = 1
    insert('# This comes from the first macro code block, number ')
    insert(code_block)
    insert("\nprint('January 1st, 2024, or later')\n")
else:
    insert('# This comes from the second macro code block, number ')
    insert(code_block)
    insert("\nprint('Earlier than January 1st, 2024')\n")

Note that pymacros4py automatically indents the insert statements and the statements code_block = ... when generating the template script, because in Python, suites of compound statements need to be indented.

This template script evaluates to:

# This comes from the first macro code block, number 1
print('January 1st, 2024, or later')

Examples for loops over text blocks:

# $$ for i in range(3):
print('Yep, i is "$$ insert(i) $$".')
# $$ :end
# $$ j = 5
# $$ while j > 3:
print('And, yep, j is "$$ insert(j) $$".')
# $$ j -= 1
# $$ :end

This template evaluates to:

print('Yep, i is 0.')
print('Yep, i is 1.')
print('Yep, i is 2.')
print('And, yep, j is 5.')
print('And, yep, j is 4.')

Example for a multi-section suite containing both indented and non-indented macro code:

# $$ for i in range(2):
print('Code from the text section, variable i is "$$ insert(i) $$".')
'''$$ # The macro code of this section is locally indented to this level,
      # but not the content of the following text literal
      more_text = """\
print('This first line is not indented.')
print('This second line is not indented.')
"""
      # We continue at the base indention, it is here
      insert(more_text)
$$'''

# $$ :end

The template script generated from this template looks roughly as follows:

for i in range(2):
    insert("print('Code from the text section, variable i is ")
    insert(i)
    insert(".')\n")
    # The macro code of this section is locally indented to this level,
    # but not the content of the following text literal
    more_text = """\
print('This first line is not indented.')
print('This second line is not indented.')
"""
    # We continue at the base indention, it is here
    insert(more_text)
    insert('\n')

This template script shows: The implementation of multi-section suites by pymacros4py meets two requirements:

• In Python, code in suites of compound statements needs to be indented. So, pymacros4py generates this indentation synthetically (re-indentation) when generating the template script.
• It must be possible to define unindented string literals. So, pymacros4py distinguishes unindented code from indented code, re-indents only the indented code, but uses the unindented code as-is.

The template evaluates to:

print('Code from the text section, variable i is 0.')
print('This first line is not indented.')
print('This second line is not indented.')

print('Code from the text section, variable i is 1.')
print('This first line is not indented.')
print('This second line is not indented.')

Def-Statement Suites Spanning Multiple Sections

If the suite of a def-statement spans multiple sections, indentation of generated results of the macro expansion is special-cased as follows:

• Macro sections: Generated code is indented as part of the calling macro section, not the defining macro section.
• Text sections: The content is also indented as part of the generated results (whereas outside the suite of a def statement, it is interpreted as literal). And the same rules apply: Zero indentation is kept, other indentation is interpreted relative to the indentation of the first content character, and the indentation is adapted to the indentation of the calling macro section.

Examples:

# $$ def some_inlined_computation(times, acc):
for macro_var_i in range('$$ insert(times) $$'):
    '$$ insert(acc) $$' = 1
# $$ :end
j = k = 0
# $$ some_inlined_computation(3, "j")
if True:
    # $$ some_inlined_computation(2, "k")

This template evaluates to:

j = k = 0
for macro_var_i in range(3):
    j = 1
if True:
    for macro_var_i in range(2):
        k = 1

Note, that the indentation of the results of the two calls of the function is defined by the indentation of the calling macro sections, and not the defining macro section. And this holds both for the macro sections and the text sections within the suite of the def statement. Like that, valid indentation is established.

Debugging

Error Messages

In case something goes wrong, pymacros4py tries to give helpful error messages.

Example: Wrong indentation within macro code

'''$$
    # first line
  # indentation of second line below base indentation, but not zero
$$'''

This template leads to the following exception:

>>> pp.expand_file_to_file("tests/data/error_wrong_indentation_in_macro.tpl.py", "out.py"
... )   # doctest: +NORMALIZE_WHITESPACE
Traceback (most recent call last):
RuntimeError: File "tests/data/error_wrong_indentation_in_macro.tpl.py", line 2:
Syntax error: indentation of line 1 of the macro code is not an
extension of the base indentation.

Example: Macro section started, but not ended

'''$$

This template leads to the following exception:

>>> pp.expand_file_to_file("tests/data/error_macro_section_not_ended.tpl.py", "out.py"
... )   # doctest: +NORMALIZE_WHITESPACE
Traceback (most recent call last):
RuntimeError: --- File "tests/data/error_macro_section_not_ended.tpl.py", line 1:
Syntax error in macro section, macro started but not ended:
'''$$
<BLANKLINE>

Example: Nesting of multi-section suites of compound statements wrong, unexpected suite end

#$$ if True:
#$$ :end
#$$ :end

This template leads to the following exception:

>>> pp.expand_file_to_file("tests/data/error_unexpected_end.tpl.py", "out.py"
... )   # doctest: +NORMALIZE_WHITESPACE
Traceback (most recent call last):
RuntimeError: --- File "tests/data/error_unexpected_end.tpl.py", line 3:
Nesting error in compound statements with suites spanning several sections,
in macro section:
  :end

Example: Nesting of multi-section suites of compound statements wrong, suite end missing

#$$ if True:

This template leads to the following exception:

>>> pp.expand_file_to_file("tests/data/error_end_missing.tpl.py", "out.py"
... )   # doctest: +NORMALIZE_WHITESPACE
Traceback (most recent call last):
RuntimeError: Syntax error: block nesting (indentation) not correct,
is :end somewhere missing?

Example: Wrong indentation of expansion results

'''$$
  insert("    # First line indented\n")
  insert("  # Second line indented, but less than the first\n")
$$'''

This template leads to an exception:

>>> try:
...     pp.expand_file_to_file("tests/data/error_result_indentation_inconsistent.tpl.py",
...                            "out.py")
... except Exception as e:
...     print(type(e).__name__)  # doctest: +NORMALIZE_WHITESPACE, +ELLIPSIS
RuntimeError

(Depending on the used Python version, the exception contains notes. If there are notes, the doctest module cannot correctly parse them. And if not, the doctest cannot handle this version-specific deviation of the results. So, above, we only check that the expected exception occurs.)

Comparing Results

Method expand_file_to_file offers an option diffs_to_result_file that returns the differences between the results of the macro expansion and the current content of the result file. If there are no differences, the empty string is returned.

Example: Showing results of a change in a template

In the following template, we changed the expression with respect to the example of section Templates and template expansion.

# $$ # In the following line, we changed the expression w.r.t. the example of
# $$ # section Templates and template expansion
# $$ v = 3 * 3
x = '$$ insert(v) $$'

Now, we compare against the result we have gotten there:

>>> print(pp.expand_file_to_file("tests/data/diff_templ_and_templ_exp.tpl.py",
...                              "tests/data/doc_templ_and_templ_exp.py",
...                              diffs_to_result_file = True))
*** current content
--- expansion result
***************
*** 1 ****
! x = 6
--- 1 ----
! x = 9
<BLANKLINE>

Viewing the Template Script

When an exception is raised during the execution of a generated template script, e.g., if there is an error in your Python macro code, the script will be automatically stored (as temporary file, with the platform specific Python mechanisms) and its path will be given in the error message.

Additionally, the method template_script of pymacros4py can be used to see the generated template script anytime.

Example: Getting the template script

>>> print(pp.template_script("tests/data/doc_templ_and_templ_exp.tpl.py")
... )   # doctest: +NORMALIZE_WHITESPACE
_macro_starts(indentation='', embedded=False,
    content_line='File "tests/data/doc_templ_and_templ_exp.tpl.py", line 1')
v = 2 * 3
_macro_ends('File "tests/data/doc_templ_and_templ_exp.tpl.py", line 1')
insert('x = ')
_macro_starts(indentation='    ', embedded=True,
    content_line='File "tests/data/doc_templ_and_templ_exp.tpl.py", line 2')
insert(v)
_macro_ends('File "tests/data/doc_templ_and_templ_exp.tpl.py", line 2')
insert('\n')
<BLANKLINE>

Here, we used the template from section Templates and template expansion. As can be seen, the real template script looks like the one shown there, but has some additional bookkeeping code that marks when macro code starts and ends during the execution of the template script.

Tracing

pymacros4py can write a trace log during parsing of a template and during execution of a template script: The options trace_parsing and trace_evaluation of method expand_file_to_file activate this functionality. We demonstrate this in the following example with method expand_file, which returns the expansion result instead of storing it to a file.

Example: Tracing of the parsing process

>>> r = pp.expand_file("tests/data/doc_templ_and_templ_exp.tpl.py",
...                    trace_parsing=True)   # doctest: +NORMALIZE_WHITESPACE
--- File "tests/data/doc_templ_and_templ_exp.tpl.py", line 1: line_block_macro:
>v = 2 * 3<
<BLANKLINE>
<BLANKLINE>
--- File "tests/data/doc_templ_and_templ_exp.tpl.py", line 2: text:
>x = <
<BLANKLINE>
<BLANKLINE>
--- File "tests/data/doc_templ_and_templ_exp.tpl.py", line 2: embedded_macro:
>insert(v)<
<BLANKLINE>
<BLANKLINE>
--- File "tests/data/doc_templ_and_templ_exp.tpl.py", line 2: text:
>
<
<BLANKLINE>
<BLANKLINE>

Example: Tracing of the evaluation process

>>> r = pp.expand_file("tests/data/doc_templ_and_templ_exp.tpl.py",
...                    trace_evaluation=True)   # doctest: +NORMALIZE_WHITESPACE
'File "tests/data/doc_templ_and_templ_exp.tpl.py", line 1': line_block_macro
>v = 2 * 3<
<BLANKLINE>
<BLANKLINE>
'File "tests/data/doc_templ_and_templ_exp.tpl.py", line 2': text
>x = <
<BLANKLINE>
<BLANKLINE>
'File "tests/data/doc_templ_and_templ_exp.tpl.py", line 2': embedded_macro
>insert(v)<
<BLANKLINE>
<BLANKLINE>
'File "tests/data/doc_templ_and_templ_exp.tpl.py", line 2': text
>
<
<BLANKLINE>
<BLANKLINE>

Changelog

v0.8.3 (2025-04-11)

• Code adapted to new, stricter tests of flake8.
• README.rst extended by chapter first examples.

v0.8.2 (2024-03-12)

• Method PreProcessor.diff and functions read_file, write_file, write_to_tempfile, and run_on_tempfile exported / added. They ease applying an external code formatter on content that has been generated by macro expansion.
• Methods import_from and insert_from support paths relative to the path of the template file, not only relative to the current directory.
• Error messages improved.
• Semantic versioning is used.

v0.8.1 (2024-02-11)

• Error messages and format of text differences improved.
• Source formatted with black default 2024.

v0.8.0 (2024-01-21)

• First published version.