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Cog is a file generation tool. It lets you use pieces of Python code as generators in your source files to generate whatever text you need.

The sections below are:

What does it do?

Cog transforms files in a very simple way: it finds chunks of Python code embedded in them, executes the Python code, and inserts its output back into the original file. The file can contain whatever text you like around the Python code. It will usually be source code.

For example, if you run this file through cog:

// This is my C++ file.
import cog
fnames = ['DoSomething', 'DoAnotherThing', 'DoLastThing']
for fn in fnames:
    cog.outl("void %s();" % fn)

it will come out like this:

// This is my C++ file.
import cog
fnames = ['DoSomething', 'DoAnotherThing', 'DoLastThing']
for fn in fnames:
    cog.outl("void %s();" % fn)
void DoSomething();
void DoAnotherThing();
void DoLastThing();

Lines with triple square brackets are marker lines. The lines between [[[cog and ]]] are the generator Python code. The lines between ]]] and [[[end]]] are the output from the generator.

When cog runs, it discards the last generated Python output, executes the generator Python code, and writes its generated output into the file. All text lines outside of the special markers are passed through unchanged.

The cog marker lines can contain any text in addition to the triple square bracket tokens. This makes it possible to hide the generator Python code from the source file. In the sample above, the entire chunk of Python code is a C++ comment, so the Python code can be left in place while the file is treated as C++ code.


Cog is designed to be easy to run. It writes its results back into the original file while retaining the code it executed. This means cog can be run any number of times on the same file. Rather than have a source generator file, and a separate output file, typically cog is run with one file serving as both generator and output.

Because the marker lines accommodate any language syntax, the markers can hide the cog Python code from the source file. This means cog files can be checked into source control without worrying about keeping the source files separate from the output files, without modifying build procedures, and so on.

I experimented with using a templating engine for generating code, and found myself constantly struggling with white space in the generated output, and mentally converting from the Python code I could imagine, into its templating equivalent. The advantages of a templating system (that most of the code could be entered literally) were lost as the code generation tasks became more complex, and the generation process needed more logic.

Cog lets you use the full power of Python for text generation, without a templating system dumbing down your tools for you.


Cog requires Python 2.7, 3.5, 3.6, 3.7, 3.8, or Jython 2.5.

Cog is installed in the usual way, except the name is “cogapp”, not “cog”:

$ pip install cogapp

You should now have a “cog” command, or in your Python scripts directory.


Cog is distributed under the MIT license. Use it to spread goodness through the world.

Writing the source files

Source files to be run through cog are mostly just plain text that will be passed through untouched. The Python code in your source file is standard Python code. Any way you want to use Python to generate text to go into your file is fine. Each chunk of Python code (between the [[[cog and ]]] lines) is called a generator and is executed in sequence.

The output area for each generator (between the ]]] and [[[end]]] lines) is deleted, and the output of running the Python code is inserted in its place. To accommodate all source file types, the format of the marker lines is irrelevant. If the line contains the special character sequence, the whole line is taken as a marker. Any of these lines mark the beginning of executable Python code:

/* cog starts now: [[[cog */
-- [[[cog (this is cog Python code)
#if 0 // [[[cog

Cog can also be used in languages without multi-line comments. If the marker lines all have the same text before the triple brackets, and all the lines in the generator code also have this text as a prefix, then the prefixes are removed from all the generator lines before execution. For example, in a SQL file, this:

--   import cog
--   for table in ['customers', 'orders', 'suppliers']:
--      cog.outl("drop table %s;" % table)

will produce this:

--   import cog
--   for table in ['customers', 'orders', 'suppliers']:
--      cog.outl("drop table %s;" % table)
drop table customers;
drop table orders;
drop table suppliers;

Finally, a compact form can be used for single-line generators. The begin-code marker and the end-code marker can appear on the same line, and all the text between them will be taken as a single Python line:

// blah blah
//[[[cog import MyModule as m; m.generateCode() ]]]

You can also use this form to simply import a module. The top-level statements in the module can generate the code.

If you have special requirements for the syntax of your file, you can use the --markers option to define new markers.

If there are multiple generators in the same file, they are executed with the same globals dictionary, so it is as if they were all one Python module.

Cog tries to do the right thing with white space. Your Python code can be block-indented to match the surrounding text in the source file, and cog will re-indent the output to fit as well. All of the output for a generator is collected as a block of text, a common whitespace prefix is removed, and then the block is indented to match the indentation of the cog generator. This means the left-most non-whitespace character in your output will have the same indentation as the begin-code marker line. Other lines in your output keep their relative indentation.

The cog module

A module called cog provides the functions you call to produce output into your file. The functions are:

cog.out(sOut=’’ [, dedent=False][, trimblanklines=False])
Writes text to the output.
sOut is the string to write to the output.
If dedent is True, then common initial white space is removed from the lines in sOut before adding them to the output. If trimblanklines is True, then an initial and trailing blank line are removed from sOut before adding them to the output. Together, these option arguments make it easier to use multi-line strings, and they only are useful for multi-line strings:
    These are lines I
    want to write into my source file.
""", dedent=True, trimblanklines=True)
Same as cog.out, but adds a trailing newline.
Prints msg to stdout with a “Message: ” prefix.
Raises an exception with msg as the text. No traceback is included, so that non-Python programmers using your code generators won’t be scared.
An attribute, the path of the input file.
An attribute, the path of the output file.
An attribute, the line number of the first line of Python code in the generator. This can be used to distinguish between two generators in the same input file, if needed.
An attribute, the text output of the previous run of this generator. This can be used for whatever purpose you like, including outputting again with cog.out().

Running cog

Cog is a command-line utility which takes arguments in standard form.

$ cog -h
cog - generate content with inlined Python code.


INFILE is the name of an input file, '-' will read from stdin.
FILELIST is the name of a text file containing file names or
    other @FILELISTs.

    -c          Checksum the output to protect it against accidental change.
    -d          Delete the generator code from the output file.
    -D name=val Define a global string available to your generator code.
    -e          Warn if a file has no cog code in it.
    -I PATH     Add PATH to the list of directories for data files and modules.
    -n ENCODING Use ENCODING when reading and writing files.
    -o OUTNAME  Write the output to OUTNAME.
    -p PROLOGUE Prepend the generator source with PROLOGUE. Useful to insert an
                import line. Example: -p "import math"
    -r          Replace the input file with the output.
    -s STRING   Suffix all generated output lines with STRING.
    -U          Write the output with Unix newlines (only LF line-endings).
    -w CMD      Use CMD if the output file needs to be made writable.
                    A %s in the CMD will be filled with the filename.
    -x          Excise all the generated output without running the generators.
    -z          The end-output marker can be omitted, and is assumed at eof.
    -v          Print the version of cog and exit.
                Control the amount of output. 2 (the default) lists all files,
                1 lists only changed files, 0 lists no files.
    --markers='START END END-OUTPUT'
                The patterns surrounding cog inline instructions. Should
                include three values separated by spaces, the start, end,
                and end-output markers. Defaults to '[[[cog ]]] [[[end]]]'.
    -h          Print this help.

In addition to running cog as a command on the command line, you can also invoke it as a module with the Python interpreter:

$ python -m cogapp [options] [arguments]

Note that the Python module is called “cogapp”.

Input files

Files on the command line are processed as input files. All input files are assumed to be UTF-8 encoded. Using a minus for a filename (-) will read the standard input.

Files can also be listed in a text file named on the command line with an @:

$ cog @files_to_cog.txt

These @-files can be nested, and each line can contain switches as well as a file to process. For example, you can create a file cogfiles.txt:


# These are the files I run through cog
myschema.sql -s " --**cogged**"
readme.txt -s ""

then invoke cog like this:

cog -s " //**cogged**" @cogfiles.txt

Now cog will process four files, using C++ syntax for markers on all the C++ files, SQL syntax for the .sql file, and no markers at all on the readme.txt file.

As another example, cogfiles2.txt could be:


template.h -D thefile=data1.xml -o data1.h
template.h -D thefile=data2.xml -o data2.h

with cog invoked like this:

cog -D version=3.4.1 @cogfiles2.txt

Cog will process template.h twice, creating both data1.h and data2.h. Both executions would define the variable version as “3.4.1”, but the first run would have thefile equal to “data1.xml” and the second run would have thefile equal to “data2.xml”.

Overwriting files

The -r flag tells cog to write the output back to the input file. If the input file is not writable (for example, because it has not been checked out of a source control system), a command to make the file writable can be provided with -w:

$ cog -r -w "p4 edit %s" @files_to_cog.txt

Setting globals

Global values can be set from the command line with the -D flag. For example, invoking Cog like this:

cog -D thefile=fooey.xml mycode.txt

will run Cog over mycode.txt, but first define a global variable called thefile with a value of “fooey.xml”. This variable can then be referenced in your generator code. You can provide multiple -D arguments on the command line, and all will be defined and available.

The value is always interpreted as a Python string, to simplify the problem of quoting. This means that:

cog -D NUM_TO_DO=12

will define NUM_TO_DO not as the integer 12, but as the string “12”, which are different and not equal values in Python. Use int(NUM_TO_DO) to get the numeric value.

Checksummed output

If cog is run with the -c flag, then generated output is accompanied by a checksum:

--   import cog
--   for i in range(10):
--      cog.out("%d " % i)
0 1 2 3 4 5 6 7 8 9
--[[[end]]] (checksum: bd7715304529f66c4d3493e786bb0f1f)

If the generated code is edited by a misguided developer, the next time cog is run, the checksum won’t match, and cog will stop to avoid overwriting the edited code.

Output line suffixes

To make it easier to identify generated lines when grepping your source files, the -s switch provides a suffix which is appended to every non-blank text line generated by Cog. For example, with this input file (mycode.txt):


cog.outl('Three times:\n')
for i in range(3):
    cog.outl('This is line %d' % i)

invoking cog like this:

cog -s " //(generated)" mycode.txt

will produce this output:

cog.outl('Three times:\n')
for i in range(3):
    cog.outl('This is line %d' % i)
Three times: //(generated)

This is line 0 //(generated)
This is line 1 //(generated)
This is line 2 //(generated)


The -n option lets you tell cog what encoding to use when reading and writing files.

The --verbose option lets you control how much cog should chatter about the files it is cogging. --verbose=2 is the default: cog will name every file it considers, and whether it has changed. --verbose=1 will only name the changed files. --verbose=0 won’t mention any files at all.

The --markers option lets you control the syntax of the marker lines. The value must be a string with two spaces in it. The three markers are the three pieces separated by the spaces. The default value for markers is “[[cog ]]] [[[end]]]”.

The -x flag tells cog to delete the old generated output without running the generators. This lets you remove all the generated output from a source file.

The -d flag tells cog to delete the generators from the output file. This lets you generate content in a public file but not have to show the generator to your customers.

The -U flag causes the output file to use pure Unix newlines rather than the platform’s native line endings. You can use this on Windows to produce Unix-style output files.

The -I flag adds a directory to the path used to find Python modules.

The -p option specifies Python text to prepend to embdedded generator source, which can keep common imports out of source files.

The -z flag lets you omit the [[[end]]] marker line, and it will be assumed at the end of the file.


Cog’s change log is on a separate change page.


I’d love to hear about your successes or difficulties using cog. Comment here, or send me a note.

See Also

There are a handful of other implementations of the ideas in Cog:

You might like to read:



I'm using Cog! I use it to do code generation for a library that's implemented in three different languages (C#, c++, Java). Linked from my blog. Thanks for the distribution fix!


This is really nice - normally I don't like the idea of having files which are hand-edited AND machine generated, but this is simple enough to change my mind about this. I really like this.

Some minor issues - the '' file in scripts has a cr-lf at the end of the first line, which makes it fail to run, and makes distutils fail to edit in the proper python bin location. Easily fixed.

Running the script with no command line parameters should cause it to print a help message.

How about an option which will cause
cog to fail with an error if the file's
actual autogenerated code doesn't match the live python output. This might make more sense than '-r' for use in a makefile, to ensure that you don't edit the wrong code and have your changes discarded.

I notice that the 'import cog' is not actually required, this is good. It would be very nice if each python snippet were run in the same global dictionary, so you could define a variable (or import a module) at one point and use it throughout.

Finally -- certain languages (VHDL,
Makefiles, Python, for instance) do
not have multi-line comments, so
you can't use it with these, unless
I am missing something. May I suggest:
if the [[[cog is preceded by some text on its line, then all lines between that
and the ]]] are checked to see if they
start with the same text, and if so, that is discarded before further processing.


Thanks, Greg. I added the non-multi-line comment support, and a few other minor things. Thanks for the suggestions!


It looks neat, but I generally like to have no codegen code in the generated code.
But a slightly different approach would fix that:

1. External Python or cog-CodeBehind file next to the source file.
2. You add naming to your cog-codegen slots.

This allows the CodeBehind codegen code to find the output slot for its generated code.

// file:MyCxxFile.hpp
/*[[[cog-slot:EnumerateMethods]]] */
void DoSomething();
void DoAnotherThing();
void DoLastThing();

// file:MyCxxFile.hpp.cogCodeBehind or MyCxxFile.hpp.cog or ...
// NOTE: Comments etc. are optional now.
import cog
fnames = ['DoSomething', 'DoAnotherThing', 'DoLastThing']
for fn in fnames:
cog.outl("void %s();" % fn)


You can come close with Cog as it is. Remember that you can import any Python module you want into the cog code. By moving all of your Python code into another module, you can reduce the Cog code to a single import statement:

// file: MyCxxFile.hpp
import MyCxxFileGen

# file:
import cog
fnames = ['DoSomething', 'DoAnotherThing', 'DoLastThing']
for fn in fnames:
cog.outl("void %s();" % fn)


Cog is great, but what I'd really like to be able to do is the following:

// [[[cog import mycodgen as m]]]
// [[[end]]]
... lots of regular code ...
// [[[cog m.somestuff()]]]
// [[[end]]]
... other code ...
// [[[cog m.otherstuff()]]]
// [[[end]]]

As it is, each cog slot seems to "forget" the globals() dict for previous cog slots (i.e. line 86 in passes an empty globals() dict to eval).


You got it: 1.3 does what you want.


Just FYI, think I've finally settled on COG as a tool for PHP code generation. Had looked at empy (some notes here: but what's clinched it is the way you've implemented the output area.

The mission is to eliminate any work PHP might do that relates to application configuration or the environment it's running in - stuff that won't change once an app is deployed so eliminate the runtime overhead.

One particular thing I want to keep is the ability to execute the PHP scripts, while hacking, before they have been run through COG, e.g.

import cog
cog.outl("require_once '/full/path/to/someClass.php';")
// While hacking, use this
require_once DEV_PATH 'someClass.php';

May even ditch the require_once completely and have COG embed the class code directly into the script.

Anyway - thanks.


Just what I'm looking for, thanks! However, am using it to generate multiple source files from each template file (ie. feed a different xml config file in to the template to generate a unique file). Is there any way to pass an argument (e.g. a file name) through the command line invocation?

That way I could provide the xml file to the template dynamically.

Thanks again!


Theo, it would be straightforward to add a -D name=value syntax to the command line. Each would create a variable in the global context. I think this would cover your needs.

I think this is a good idea. I'll add it into Cog soon.


Excellent, looking forwards to it,


You know what COG really needs? A "COG-recipes" site. I'm sure people have developed some interesting scripts.

One script I'm interested in (that I'm sure others would be too) is a script that will generate C++ functions that will translate enum values to and from string representations. Having a recipes site would allow me to share my code as well as allow others to give feedback about my script.


can you use COG to make codes out of videos andpictures and soundfile? for instance on myspace you can go to websites and get html codes for videos, wich resemble COG in a way, would you be able to e-mail me backon this subject?


I like the idea expressed by Kevin above about a "COG recipes" site. Because of that I started a COG Wiki site ( to allow for discussions and code snippets for COG.

Anyone who has snippets to share, please post them. The Wiki will become better as more people share their code.


Hey man,
this is a really handy tool you wrote.
Keep up the great work.

*And why not directing the stdout instead of cog.outl ?



You should note that intalling COG also requires the PATH module.


Hi, this tool certainly looks very promising, I'm going to give it a go to clean up some of the internals used in aqsis ( The other devs favour xsltproc at the moment - I'll see if I can convince them and myself...

One thing I noticed after just installing cog-2.0 from the tarball is that cog.out no longer seems to recognise the trimnewlines command. After grepping the source I see that it's apparently been changed to trimblanklines?


is anyone willing to try and teach me this stuff ? i find it really interesting.. but i don't expect anyone to take me up on this, it seems really complicated. thats why im so interested.


Bug report.

import cog

cog.outl(" extern void simple1();")
cog.outl(" extern void simple2();")

I think the result should be indented, since the dedent parameter isn't given and the default value is False.

However, the result is "dedented". (reindentBlock is called twice)


Jay: this code is behaving as intended. I've edited the description of indentation to try to make it clearer. The dedent parameter doesn't affect the indentation of the line in the output, just the interpretation of a multi-line string parameter. Cog collects output as a block of text, then indents the block to match the generator. The dedent parameter affects how cog adds the lines to the collected output, but now how that output is finally written.

To see what the dedent parameter does, try putting five spaces at the left of one of your lines, and try it with dedent=True and dedent=False to see the difference.

Sorry for the confusion. If you need the output block indented differently, indent your entire cog block to where you want the output to go.


Hi Ned,

This is some neat python app :-)

I would like to try it for some Java apps around here. Has anyone tried whether
it works with jython? If so I could easily use it from ant and would not
require people to install Python.


... I just checked, it seems that jython won't work since cog requires the compiler package (which isn't available on jython).


I've never used Jython, but yes, Cog depends on being able to execute the chunks of Python code it finds.


nice tool

glen worstell 7:35 PM on 27 May 2009

I happily used cog 3 or so years ago on a C# project. Now I need it again! Thanks...

FYI, I have code that users must run, and the code has strings with passwords in them. I don't want users to be able to find passwords by looking at the code they run (strings show up in binary code). I'll use cog to generate encrypted strings, and when the program runs the strings will be decrypted. Users won't be able to see clear text passwords no matter what they do, except if they figure out the decryption code. This is not NSA type security, it is for ordinary users.

Of course I could manually encrypt the password strings; then I'd not need cog. But that is a lot more work as there are several programs that will use this technique, and passwords change from time to time. The python code will be very short.


chandra sekhar katuri 3:54 AM on 26 Jun 2009

Great tool

Haroldo Stenger 4:16 AM on 4 Oct 2009

Ned, I love cog. I use emacs too, and run cog from within emacs with a single keystroke, as you noted here

thank you very much


It saved me a lot of work.



I prefer Enhanced CWEB and yesweb to do some of these things. (Enhanced CWEB will generate #line directives in the output file, and you can use metamacros and named chunks to do all sorts of things. This program can be used with C and C++.) (yesweb is written entirely in TeX, so you can use any TeX commands to extend its features.)

Here are a bunch of examples of Enhanced CWEB metamacros:

@m Repeat@T

@m _Repeat@T

@m Repeat@W
Bq``: zyqnB" times:\ @>

``u{YJ"@<Predeclaration of procedures@>=
"@<Procedure codes@>=
B" {
``u{YJ"@<Static declarations@>= static
"@<Procedure codes@>= static
B" {

j/@t$\flat\ $@>

``k"@<Data of kinds@>=
"#define kind (kind_data[_kindnum])
"#undef _kindnum
:#z:dz:ez:fz:iz:nz:ez: z:_z:kz:iz:nz:dz:nz:uz:mz: z



: zBqA


I have code that has Windows line endings and got syntax errors on Linux because of them. Other developers use only Windows so changing the line endings of the file is a bit too tedious. To fix this, I had to modify the evaluate method to convert \r\n line endings to \n before running the code:

intext = "import cog\n" + intext + "\n"
intext = intext.replace("\r\n", "\n"); # Replace Windows line endings


I like it. I use it.
I would like the ability to add comments in my C code (like "do not edit" or "generated from somefile.txt"), which is a bit awkward since the closing comment mark terminates the cog code.

I added this to my local version, which would be nice in a future release:

---	(revision 2416)
+++	(revision 2417)
@@ -131,6 +131,7 @@
         cog.cogmodule.msg = self.msg
         cog.cogmodule.out = self.out
         cog.cogmodule.outl = self.outl
+        cog.cogmodule.c_comment = self.c_comment
         cog.cogmodule.error = self.error
         self.outstring = ''
@@ -167,6 +168,13 @@
         self.out(sOut, **kw)
+    def c_comment(self, sOut='', **kw):
+        """ Add a C-style comment to the output
+        """
+        self.out('/* ')
+        self.out(sOut, **kw)
+        self.out(' */\n')
     def error(self, msg='Error raised by cog generator.'):
         """ The cog.error function.
             Instead of raising standard python errors, cog generators can use


@Brad: I would rather not add a language-specific method like c_comment. Couldn't you output the comment closer as two strings, or with a backslash: cog.outl("/* comment *\/")


@Ned: Sure. I'm ok with keeping it as a local change - just thought it might be an interesting addition.

Splitting the comment isn't very "attractive", which is why I went with a wrapper (not dissimilar to how you don't _really_ need outl() - you could just use out() and add a "\\n" each time.)

Thanks again for providing cog!


When calling, it seems that it passes in sys.argv and modifies it. We like to do things locally that rely on sys.argv (mainly passing in the name of a program to databases so we know what/who's logged in).

Would you consider either having pass in list(sys.argv) to pass in a copy of sys.argv, or in set argv0=argv[0];argv_rest = argv[1:]; and then use argv_rest for parsing instead of doing argv.pop(0)?


Sorry for asking stupid question. Does this example still work?

import cog
cog.outl(" extern void simple1();")

I am having trouble running this in Eclipse/Pydev. Thanks for your help.

Roger Alexander 11:02 AM on 27 Jun 2011

A useful feature would be to have an option that would allow the output of running cog to accumulate between ]]] and [[[end]]] rather than have it deleted. An example use case for this would be a file whose purpose is to accumulate data that is produced periodically, such as from a weather data generation service. This would be a great application for cog is there were someway to have it not deleted what was generated from previous runs. Further, it would be useful to specify that this behavior should per block rather than per file. This perhaps could be implemented as a meta-variable on the opening delimiter, say something line "p[[[cog{retain}", or something to that effect.

What do you think?


Hi Ned,
A very useful tool.
I would like to know whether, if I import one of my module in one file, then can I use the functions present in that module or any python variable defined in that cog file in other file ?

Spencer Rathbun 12:43 PM on 19 Sep 2011

Hi Ned,

I'm using cog for automating some code generation from a text file, and I love it. It works great for removing some of the repetition from my day. Unfortunately, I've come across a bug. I read my input text file, and store my code lines in a dictionary of lists.

When I come to a specific section I loop over each line in each list, in the appropriate dictionary entry, and print them with cog.outl(). All the lines start with \t to indent them appropriately, but cog strips this out, unless I also print a line with no indent.


I have found a bug (Might not be after all).
Currently with the latest COG available for download, I am unable to indent the code with '\t'(tab) in the beginning.
I am using cog.out and cog.outl for creating the code statements.
Currently I have changed the "reindentBlock" function in "".
Changed statement to "l = l.replace(oldIndent, '\t', 1)".
Can you please check if it is a bug or am I missing something ?


Your "Success Story" link was changed on you. It is now .


@Jason: thanks, fixed!


Great!, this is what I found.
inline template is very interesting to maintain codes.


Thanks for the useful tool!

It would be helpful if you added an example for how to use -D parameters. It took me a while to realise that they are visible to the code in the template, but not to the imported code, i.e. I needed to pass in the globals as parameters to my generator code.

Brecht Machiels 9:24 AM on 2 Jul 2013

Very useful tool... thanks! Using it for C++ code generation.

Please consider changing line 637 in to (and importing glob) to allow for wildcards on Windows:

            for filename in glob.glob(args[0]):

Jean-François Giraud 8:34 AM on 7 Feb 2014

Thanks for this tool!

It would be usefull to automatically create directory when option -o is used.


I use cog in my C++ project to encrypt data. And I've got a question.. Does cog work with unicode? The problem is that I try to get character code in unicode project. On my individual python project it works:

# -*- coding: cp1251 -*-
k = ord('а') #russian unicode symbol, k = 1072

But cog gaves me ASCII code

#-*- coding: cp1251 -*-
import cog

def test() :
k = ord('а')
cog.out('CString strChar = _T("')
cog.out("".join("%d" % ord(k)))

and it gaves me k = 224 # the same russian symbol but in ASCII

I use the same interpretator in both cases (IronPython 2.7). Can you give me an advice?


Hi. Awesome tool!

Small request: possible to add option to set verbosity, with following possibilities:
- same as now, show changed files, and unchanged files
- show only changes files
- no output at all, completely silent

The advantage is that currently, I've added it to my cmake file, with a wildcard, like ' *.h', and I'd prefer to only be notified when something change.d


By the way, I used cog to generate my header file declarations:
- created a python module '':
- then simply add a small cog declaration to each of my header files, like

Nickolay Bukreyev 2:56 PM on 9 Feb 2015

; [[[cog
; cog.outl("I've found a bug")
; ]]]
; [[[end]]]
This code will result in a syntax error if it is placed in the beginning of a file encoded in UTF-8. The first line starts with '<BOM>; ' while the others just with '; ', therefore, semicolons are not removed.

You'd better open all files with encoding="utf-8-sig" to avoid such untrackable bugs.

Oh, and I nearly forgot. Cog is awesome!


Awesome tool! The only serious limitation I struggle with is the missing ability to "protect" parts of the generated code.
Typically, one may want to generate a skeleton that they can then fill-in with manually written code. If the generation is re-run, the manually written code should be preserved. Do you you think you could add such a feature?
Thanks for the amazing piece of code you wrote! The "dedent" feature alone is worth the price of the ticket ;-)


@Ned, is there a way to import a file with embedded cog data into another file?
That is, as you would `import cog` or `import MyModule`, be able to `import MyFile.h` (obviously not using the import keyword though) so that variables in the cog section of the imported file are available to the cog in the current file?


@Steve: there is no way to do that, and you are the first to ask about it. I would make a .py file containing the common stuff, and import that into each cogged file that needed it.


@Ned, thanks for the reply.

Including a common .py file doesn't work very well for my particular use-case, which is to do a mash-up of part-generated (à la protobuf) and part-hand-written message structs.

Cog is used for generating the serialisation code, whilst allowing hand-written business logic to live alongside.

What I'm trying to do is add a validation part, where only serialisable types and Cog-generated serialisable message structs can be using in new serialisable messages (ie: you can import another message struct if you want it have it as a member)

At the moment I'm leaning towards wrapping a call to `callableMain` or `processOneFile`? I am obviously somewhat reticent to be using Cog internals when they are not part of the stable interface.

Perhaps just the price I will have to pay...

If I get a decent version working which isn't too hacky, I could submit it for review to see if you think it worthwhile to include as a feature?


I can't get cog to run! I get "'cog' is not recognized as an internal or external command, operable program or batch file."

I did: python install

I can find in my scripts dir:

Any ideas for how to get it to work?


Bruce: If you ever come back, the command is ""


To run it you would have to do: python


Actually, more recently I installed cog using pip on a new computer and it worked no problem - no idea why it wasn't working before!


What do you think about adding a possibility
to generate C type definition based on a ctypes class?


import cog
import ctypes

class Numbers(ctypes.Structure):
_fields_ = [
('a', c_ubyte),
('a', c_ubyte)



struct Numbers {
unsigned char a;
unsigned char b;


How to read the var from

// file: MyCxxFile.hpp
import MyCxxFileGen

cog -D var=12 MyCxxFile.hpp


@hawk_king: I'm not going to add a ctypes-specific feature to Cog. It does one simple thing, and that is it. If you write a ctypes structure generating function, you can invoke it from your Cog template if you like.

The values you define with -D are available as globals in your Python code. Just use the name "var".

Stanislav Ermakov 9:11 PM on 29 Dec 2015

It supposed to work relative slow?
Any issues about speed you know?

It look like it spent a 0.5 sec per file with "hello world" example.



Hi Ned, a few days late but still happy new year! I just installed cog in Windows for python 3.5. During installation I got a syntax error saying that in line 65 "0644" is an invalid token. I googled and it is because that from python 3 "0o644", instead of "0644", should be used. Just a remark. Thank you very much for such a nice tool.


I use COG to generate two files, a .c and a .h. I have a common python module that initializes some data for the code generation.
Is it possible somehow that this python module shall be called once? Now I have to import it in both of my templates.


Excellent tool. Although I'm having trouble integrating it with Make. Is there a way to retain a leading tab character in



Nice tool! Is there an option to not delete any existing generated code? That way, I could iterate with cog to add generated code to existing generated code.



Oops, I didn't notice the 'cog.previous' attribute. That's perfect, thanks!


Hey, I really like the idea of this tool. But for me it doesnt work.
I have multiple cog-blocks in a file, and only the first one gets executed.
Do i need an extra parameter to tell, that cog that there are multiple blocks per file?


I found the problem.
I used a module which defined some lists, and also imported everything from cog, and then i used outl() from my module. importing cog seperatly solved my problem

Marek Lewandowski 7:02 PM on 23 Jul 2017



These are lines I
want to write into my source file.
""", dedent=True, trimblanklines=True)
These are lines I
want to write into my source file.
and **dedent=False**


These are lines I
want to write into my source file.
""", dedent=False, trimblanklines=True)
These are lines I
want to write into my source file.

This example is in documentation and don't work.

Alexey Ivakin 1:41 PM on 11 Dec 2017

I am completely new to Python, so maybe others did figure it out on their own and I can't.

I installed Cog and tried one example, which failed miserably. I figured out that Cog depends on "handyxml" and the latter depends on PyXML.

Since PyXML is not available anymore, how to use handyxml and make Cog running properly using currently available tools?

Which modules to install and which modifications to make?

P.S. Sorry for posting this question both on handyxml page and here, but I'm not sure if both of them are monitored nowadays, if any.


@Alexey: Cog doesn't depend on handyxml. The dependency was removed in version 2.2, over eight years ago. You shouldn't need it.

Alexey Ivakin 7:56 AM on 12 Dec 2017

I used example from Python success story and it uses handyxml to parse xml "property" field:
/* [[[cog
import cog, handyxml
for p in handyxml.xpath('Properties.xml', '//property'):
cog.outl("Property%s," %
]]] */

So instead of importing handyxml I need to import some other module and use that module's methods for reading an XML file? Or is there some built-in methods not requiring other modules?


@Alexey you don't have to use the same modules the example does. Find modules that work for you.


Hello Ned,

How can i use / invoke cog if i need some kind of wrapper around it (or in other words: how does one use cog from within another python-module)?

The thing is e.g. i need to extract some data from a json-file as a pre-step which should then be cross-referenced within the python-code of the sourc-code-template-files.

Therefore i need to write my own module which reads in mentioned json-data and then should invoke cog, e.g.:

1) within some own python-module:
def pre_step()
# read in json-file / data here into some data-structure
data = ...

2) within that same module invoke cog on e.g. a file test.h (how to do that?) Examples above are using it only from command line. Of course, an exex(..) or other system-call possibility is there, but is cog only intended to be used via command-line? I would have to prepare any needed data within the template itself then, or?

Thnx alot.

Walid Shouman 9:57 PM on 25 Dec 2017

I find cog really useful, thanks a lot for creating it, I'm using it for some Blender addon experimentation.

Is it possible to have the place holders (ie: %s be accompanied with the variable directly after it).

Current implementation is:
cog.out("codeblock %s ... %s ... %s code block" (% var1 var2 var3))

Suggested implementation is:
cog.out("codeblock %s{var1} ... %s{var2} ... %s{var3} codeblock")

or even could be combined like:
cog.out("codeblock %s{var1} ... %s ... %s{var3} codeblock" (% var2))

I find putting the variables in place will be very handy sometimes, that would save me from counting to 10 place-holders to know which variable to update.

Thanks a lot.


@Walid: if you can use Python 3.6 or later, you can use f-strings:

cog.out(f"codeblock {var1} ... {var2} ...etc")

If you need to use an earlier Python, you can do this:

cog.out("codeblock {var1} ... {var2}".format(**vars()))


Dear Ned!
I am trying to use Cog on code than might need some regeneration, and I don't see a way to let programmer write his code into part that is generated inside a loop.
For (oversimplified) c example I want to create 3 classes, and let programmer expand them after generation. Next I want to generate 4th class, and preserve what programmer has written into 3 already existing.
*Classes = ['A', 'B', 'C']
*for class in Classes
* cog.outl(f"class {class} ")
* cog.outl(f"{")
* cog.outl(f"public:")
* cog.outl(f"{class}(); ")
* cog.outl(f"") # here programmer wants to put his code.
* cog.outl(f"}")


hmm, Maybe there is clever workaround for this using git? It will easily point out all lines deleted by generator, but I still don't see a way to minimize human engagement.


@Adam, it sounds like you would need to move the generated code out of the cog clause, so that it isn't manipulated any more, but that sounds awkward. It might be that you are looking for a different tool: like cookiecutter, for templating out an initial set of files that are then hand-edited?

Eric Holmberg 12:23 AM on 7 Apr 2018

I'm working on a cython development and finding COG is Fabulous at reading in header files and generating cython/python code. But ...our cross platform development pipeline uses yocto, so i need a yocto cog recipe so cog can run under bitbake. Does anyone have suggestions on how to create this?

Alfonso Pinto 3:26 PM on 11 May 2018

Hi @Ned,

First, let me thank you for this awesome piece of software.

I think I understand @Adam request as I may facing some similar issue. Let me explain:

I like one idea for C development that is presented in a book called "Object-Oriented Programming with ANSI-C". It basically describes a way to use OO paradigm with plain C.

The methods described in the book requires a lot of boilerplate, mainly copying 2-3 files and repeating some structures and some patterns in the class files the developer wants to create.

The book presents some kind of preprocessor to create the code but I don't like the idea as the files the developer has to create doesn't resemble C, it's like using a new DSL.

What I'm doing is creating a CLI program that allows to create a new project, create new classes, adding methods... This information is stored in a json file and then I use cog to generate the final files.

Alfonso Pinto 3:28 PM on 11 May 2018

At the end, the class files contains the required boiler plate plus a skeleton of the methods to implement.

The issue comes when you want to add a new method or update the boilerplate and want all the created files to be updated. If I run cog again, the implementation added by a developer will be removed and the class files will be left again with the skeleton methods.

Sorry for the long 2 posts

Eric Holmberg 3:53 PM on 11 May 2018

Update to my previous post about a yocto bitbake recipe for cog - i gen'd one, let me know if you would like me to share it.


Awesome. Thanks for the easy-to-use tool!

We import and call function from ouside python module, but it's so easy with the cog.


Hi Ned,
thanks for cog, looks pretty cool!
I have same question as @Efendi above, not sure doing this is possible?


Hi Ned,

Just wanted to say thanks for this tool.

I'm using it autogenerate intialization tables for embedded C micro controller code. I like the way you have kept the tool language agnostic, and tied in the ever expaniding power of python. I think this tool will live on for a long time!

Best Wishes,

Brian Moran


What about the recipes site? :)

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