snippetpythonMinor

Parse OpenFoam files in Python

Submitted by: @import:stackexchange-codereview·Mar 10, 2026·

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openfoamfilespythonparse

Problem

I'm writing a Python parser for OpenFoam mesh files. When the mesh is big, I face performance issues.

Here is the format of the file describing the points:

/--------------------------------- C++ -----------------------------------\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.2.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\---------------------------------------------------------------------------/
FoamFile
{
    version     2.0;
    format      ascii;
    class       vectorField;
    location    "constant/polyMesh";
    object      points;
}
//                                     * //

10
(
(2.14633 0.955 -0.627026)
(2.14633 1.005 -0.627026)
(4.0935 0.955 -0.389604)
(4.0935 1.005 -0.389604)
(0.199157 0.955 -0.864447)
(0.199157 1.005 -0.864447)
(3.075 1.005 0.562347)
(3.11114 1.005 0.558563)
(3.075 0.955 0.562347)
(3.11114 0.955 0.558563)
)

// *** //

The file describing the tetrahedron points:

`/--------------------------------- C++ -----------------------------------\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.2.0 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\---------------------------------------------------------------------------/
FoamFile
{
version 2.0;
format ascii;
class faceList;
location "const

Solution

A few suggestions:

Follow pep8. Your code is pretty good already, but your naming in particular is not correct.

Although I can see the advantage of grouping everything into a class, the individual file parsers are not directly dependent on the class. I would split those out into their own functions. This will make testing in particular easier. You can then have wrapper methods in the class that call the parser functions.

You are loading every file completely into a list. This takes a lot of memory for large files. Worse, it requires parsing the entire list twice, once for the header and once for the body. This is likely a big source of your performance issues. It would be much more memory-efficient to iterate over the lines in the file. I would recommend turning the reader into a generate that takes an iterator (which will usually be the file, but could be arbitrary lists of strings for testing), does the stripping, yields the stripped line, and skips empty lines. This has the added advantage that it will keep track of your progress, so you don't need to go back and read through the header again when you want to parse the body.

If you use a generator, you can create a for loop that runs until it reaches the part you want, then breaks, then you can have a second for loop that picks up where the first left off. This will greatly reduce the number of if tests you have to do.

You are parsing the list of numbers yourself. Don't, numpy has a fromstring function that can parse a string to a numpy array for you. It is much faster than your approach. This is also likely a major source of performance issues.

You should always use with for opening files. This safely closes them even when there is an error. In the default Python version, files are closed automatically when the function exits, but that doesn't necessarily happen in other Python implementations like Pypy. It is much safer to use with to close the files exactly when you intend to.

You can use tuple unpacking to split the lines for your dicts. So key, value = el.split().

You create a class, but then parse everything into a single dict that holds everything. That defeats the purpose of having a class. You should either parse the components into class attributes, or you should just use functions and return a single dict.

You hard-code the file names. I would make the file names arguments, with the default argument being the default name.

rep is a directory, not a repetition. The repetition may be in the directory name, but there is no reason it has to be. So this is stylistic, but I would call it dirname or something. There is no reason to mentally limit how you organize your files like that.

You make all the parsers subfunctions of __init__. This again defeats the purpose of having a class. They should be methods.

Your classes should derive from object.

So overall, here is how I would structure the code:

```
def clean_lines(lines):
for line in lines:
line = line.strip().strip(';')
if not line:
continue
yield line

def consume_lines(lines, targ):
for line in lines:
if line == targ:
return

def header_parser(lines):
consume_lines(lines, '{')

res = {}
for line in lines:
if line == '}':
break
key, value = line.split(maxsplit=1)
res[key] = value.strip('"')

return res

def boundary_parser(lines):
consume_lines(lines, '(')

sections = {}
for line in lines:
if line == ')':
break
if line != '{':
name = line
sections[name] = {}
continue
for subline in lines:
if subline == '}':
break
key, value = subline.split(maxsplit=1)
sections[name][key] = value

return sections

def points_parser(lines):
consume_lines(lines, '(')

points = []
for line in lines:
if line == ')':
break
points.append(np.fromstring(line[1:-1], sep=' '))
return points

def faces_parser(lines):
consume_lines(lines, '(')

faces = []
for line in lines:
if line == ')':
break
faces.append(np.fromstring(line[2:-1], dtype=np.int32, sep=' '))

return faces

class ParsedMesh(object):
def __init__(self, dirname):
self.dirname = dirname
self.parse_boundary_file()
self.parse_points_file()
self.parse_faces_file()

def _parser(self, parser, fname, dirname):
if dirname is None:
dirname = self.dirname
if dirname:
fname = os.path.join(dirname, fname)
with open(fname) as fobj:
lines = clean_lines(fobj)
header = header_parser(lines)
parsed = parser(lines)
return parsed, header

def parse_boundary_file(self, fname='boundary', dirname=None):
self.boundary

Code Snippets

def clean_lines(lines):
    for line in lines:
        line = line.strip().strip(';')
        if not line:
            continue
        yield line


def consume_lines(lines, targ):
    for line in lines:
        if line ==  targ:
            return


def header_parser(lines):
    consume_lines(lines, '{')

    res = {}
    for line in lines:
        if line == '}':
            break
        key, value = line.split(maxsplit=1)
        res[key] = value.strip('"')

    return res


def boundary_parser(lines):
    consume_lines(lines, '(')

    sections = {}
    for line in lines:
        if line == ')':
            break
        if line != '{':
            name = line
            sections[name] = {}
            continue
        for subline in lines:
            if subline == '}':
                break
            key, value = subline.split(maxsplit=1)
            sections[name][key] = value

    return sections


def points_parser(lines):
    consume_lines(lines, '(')

    points = []
    for line in lines:
        if line == ')':
            break
        points.append(np.fromstring(line[1:-1], sep=' '))
    return points


def faces_parser(lines):
    consume_lines(lines, '(')

    faces = []
    for line in lines:
        if line == ')':
            break
        faces.append(np.fromstring(line[2:-1], dtype=np.int32, sep=' '))

    return faces


class ParsedMesh(object):
    def __init__(self, dirname):
        self.dirname = dirname
        self.parse_boundary_file()
        self.parse_points_file()
        self.parse_faces_file()

    def _parser(self, parser, fname, dirname):
        if dirname is None:
            dirname = self.dirname
        if dirname:
            fname = os.path.join(dirname, fname)
        with open(fname) as fobj:
            lines = clean_lines(fobj)
            header = header_parser(lines)
            parsed = parser(lines)
        return parsed, header

    def parse_boundary_file(self, fname='boundary', dirname=None):
        self.boundary, self.boundary_hdr = self._parser(boundary_parser,
                                                        fname=fname,
                                                        dirname=dirname)

    def parse_points_file(self, fname='points', dirname=None):
        self.points, self.points_hdr = self._parser(points_parser,
                                                    fname=fname,
                                                    dirname=dirname)

    def parse_faces_file(self, fname='faces', dirname=None):
        self.faces, self.faces_hdr = self._parser(faces_parser,
                                                  fname=fname,
                                                  dirname=dirname)

Context

StackExchange Code Review Q#101191, answer score: 4

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