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This is a Tcl extension to effortlessly to call bidirectionally between Tcl and Python, targeting Tcl >= 8.5 and Python 3.6+

The extension is available under the 3-clause BSD license (see "LICENSE").

Tcl users may also want to consider using pyman, a Tcl package that provides a higher level of abstraction on top of tclpy.


You can import the libtclpy in either a Tcl or Python parent interpreter. Doing so will initialise an interpreter for the other language and insert all libtclpy methods. This means you can call backwards and forwards between interpreters.


General notes:

  • Unless otherwise noted, 'interpreter' refers to the python interpreter.
  • All commands are run in the context of a single interpreter session. Imports, function definitions and variables persist.
  • Exceptions in the python interpreter will return a stack trace of the python code that was executing. If the exception continues up the stack, the tcl stack trace will be appended to it. They may be masked (as per tcl stack traces) with catch.


  • py call ?obj.?func ?arg ...?
    • takes: name of a python function
    • returns: return value of function with the first appropriate conversion applied from the list below:
      • None is converted to an empty string
      • True is converted to 1
      • False is converted to 0
      • Python 'str' objects are considered to be byte arrays
      • Python 'unicode' objects are considered to be unicode strings
      • Python 'number' objects are converted to base 10 if necessary
      • Python mapping objects (supporting key-val mapping, e.g. python dicts) are converted to tcl dicts
      • Python sequence objects (supporting indexing, e.g. python lists) are converted to tcl lists
      • Otherwise, the str function is applied to the python object
    • side effects: executes function
    • func may be a dot qualified name (i.e. object or module method)
  • py eval evalString
    • takes: string of valid python code
    • returns: nothing
    • side effects: executes code in the python interpreter
    • Do not use with substituted input
    • evalString may be any valid python code, including semicolons for single line statements or (non-indented) multiline blocks
    • errors reaching the python interpreter top level are printed to stderr
  • py import module
    • takes: name of a python module
    • returns: nothing
    • side effects: imports named module into globals of the python interpreter
    • the name of the module may be of the form module.submodule

example tclsh session:

% load ./
% py eval {def mk(dir): os.mkdir(dir)}
% py eval {def rm(dir): os.rmdir(dir); return 15}
% py import os
% set a [py eval {print "creating 'testdir'"; mk('testdir')}]
creating 'testdir'
% set b [py call rm testdir]
% py import StringIO
% py eval {sio = StringIO.StringIO()}
% py call sio.write someinput
% set c [py call sio.getvalue]
% py eval {divide = lambda x: 1.0/int(x)}
% set d [py call divide 16]
% list [catch {py call divide 0} err] $err
1 {ZeroDivisionError: float division by zero
  File "<string>", line 1, in <lambda>
----- tcl -> python interface -----}
% py import json
% py eval {
def jobj(*args):
    d = {}
    for i in range(len(args)/2):
        d[args[2*i]] = args[2*i+1]
    return json.dumps(d)
% set e [dict create]
% dict set e {t"est} "11{24"
t\"est 11\{24
% dict set e 6 5
t\"est 11\{24 6 5
% set e [py call jobj {*}$e]
{"t\"est": "11{24", "6": "5"}
% py import sqlite3
% py eval {b = sqlite3.connect(":memory:").cursor()}
% py eval {def exe(sql, *args): b.execute(sql, args)}
% py call exe "create table x(y integer, z integer)"
% py call exe "insert into x values (?,?)" 1 5
% py call exe "insert into x values (?,?)" 7 9
% py call exe "select avg(y), min(z) from x"
% py call b.fetchone
4.0 5
% py call exe "select * from x"
% set f [py call b.fetchall]
{1 5} {7 9}
% puts "a: $a, b: $b, c: $c, d: $d, e: $e, f: $f"
a: , b: 15, c: someinput, d: 0.0625, e: {"t\"est": "11{24", "6": "5"}, f: {1 5} {7 9}



  • tclpy.eval(evalstring)
    • takes: string of valid Tcl code
    • returns: the final return value
    • side effects: executes code in the Tcl interpreter
    • Do not use with substituted input
    • evalString may be any valid Tcl code, including semicolons for single line statements or multiline blocks
    • errors reaching the Tcl interpreter top level are raised as an exception

example python session:

>>> import tclpy
>>> a = tclpy.eval('list 1 [list 2 4 5] 3')
>>> print a
1 {2 4 5} 3


It is assumed that you

  • have got the repo (either by git clone or a tar.gz from the releases page).
  • have updated your package lists.

The build process fairly simple:

  • make sure make and gcc are installed.
  • make sure you can run python-config and have the Python headers available (usually installed by the Python development package for your distro).
  • locate the file and make sure you have the Tcl headers available (usually installed by the Tcl development package for your distro).
  • run make
    • specifying the path if not /usr/lib/ (TCLCONFIG=/path/to/
    • disabling tcl stubs if you wish to use Python as the parent interpreter (TCL_STUBS=0). Note this then requires compilation per Tcl interpreter.

On Ubuntu the default path is correct:

$ sudo apt-get install -y python-dev tcl-dev
$ cd libtclpy
$ make

For other distros you may need give the path of E.g. CentOS 6.5:

$ sudo yum install -y python-devel tcl-devel make gcc
$ cd libtclpy
$ make TCLCONFIG=/usr/lib64/

Now try it out:

$ TCLLIBPATH=. tclsh
% package require tclpy
% py import random
% py call random.random


Run the tests with

$ make test


  1. Be very careful when putting unicode characters into a inside a py eval call - they are decoded by the tcl parser and passed as literal bytes to the python interpreter. So if we directly have the character "ಠ", it is decoded to a utf-8 byte sequence and becomes u"\xe0\xb2\xa0" (where the \xXY are literal bytes) as seen by the Python interpreter.
  2. Escape sequences (e.g. \x00) inside py eval may be interpreted by tcl - use {} quoting to avoid this.


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