I work on a code-base that uses xml to set up problems and specify model parameters. I've created a script that I run in tandem with our code. This script will store important model information parsed from the most recent xml file and eventually end up in a LaTeX document. This script will help me keep track of model parameters I've tried and aid in reproducibility.

One problem I've come across is that, as I change model parameters, certain nodes will be deleted from the xml file and cause my script to crash. Instead, I've created a solution that will attempt to parse what I want, but if it doesn't find it, it will just return an empty dictionary.

This leads me to merging a bunch of dictionaries and I'm not quite sure this is the most idiomatic/efficient way. For this code-review I would like any feedback on how to approach this problem better, plus any styling or formatting suggestions.

Here is a sample xml file ./low_tax/il_train.xml:

    <ROM name="arma" subType="ARMA">
      <Fourier>8760, 2190, 168, 24, 12, 8, 6, 3</Fourier>
      <Segment grouping='interpolate'>
        <subspace pivotLength='168' shift='zero'>HOUR</subspace>

Here is the python code:

#!/usr/bin/env python3
from pathlib import Path
import xml.etree.cElementTree as ET
from datetime import datetime

def search_node(root: ET.Element, node: str, children: list) -> dict:
    Return dictionary containing information requested from node children.

    @In: root, ET.Element, root node of xml tree.
    @In: node, str, a string containing xpath to parent node of interest.
    @In: children, list, a list of expected children nodes.
    @Out: dict, a dictionary containing retrieved information for node.
    node_str = node + "/{child}"
    values = {
        # This information will be placed in LaTeX table;
        # Therefore, we need to preemptively escape underscores.
        k.replace("_", "\_"): root.findtext(node_str.format(child=k)) for k in children
    return values

def parse_xml(xml_file: Path) -> dict:
    Parse model information from xml file.

    @In: xml_file, Path, path to current specified xml_file.
    @Out: dict, a dictionary of information parsed from xml.
    root = ET.parse(xml_file).getroot()
    now = datetime.now().strftime("%Y-%m-%d %H:%M:%S.%f")[:-7]

    # Information parsed from xml file.
    case_info = {
        "state": xml_file.name.split("_")[0].upper(),
        "strategy": xml_file.resolve().parent.name,
    model_info = search_node(root, "Models/ROM", ["P", "Q", "Fourier"])
    model_info = {**model_info, **root.find("Models/ROM/Segment/subspace").attrib}
    pp_info = search_node(
        ["SKLtype", "n_clusters", "tol", "random_state"],
    samp_info = search_node(root, "Samplers/MonteCarlo/samplerInit", ["limit"])
    misc_info = {"created": now}

    # Merge all dictionaries
    # This should allow us to not fail on missing nodes
    info_dict = {**case_info, **model_info, **pp_info, **samp_info, **misc_info}

    # Drop any keys with None values to filter the table
    filtered = {k: v for k, v in info_dict.items() if v is not None}
    return info_dict

if __name__ == "__main__":
    xml_file = Path("./low_tax/il_train.xml").resolve()
    model_info = parse_xml(xml_file)
  • \$\begingroup\$ which docs parser do you use for the @In/@Out type parameter description? \$\endgroup\$
    – hjpotter92
    Commented Oct 14, 2020 at 20:37
  • \$\begingroup\$ I'm pretty sure it's supposed to be doxygen but our documentation tools have fallen severely behind. I think it broke a while back and we haven't had the funding to fix it. A mess I know :/ \$\endgroup\$
    – dylanjm
    Commented Oct 14, 2020 at 20:44

2 Answers 2


Nice, type hints! One thing I might start with, after working with a strict mypy configuration, is to make the type declarations stricter, and then enforce that strictness. For example, a type of list is equivalent to list[Any]. In the case of the children parameter you know more than that: it's list[str]. mypy has an option to disallow "any" generics.

In the same vein you can use TypedDict to specify the types of the contents of your dicts. You can specify total=False if some of the entries in the dict are optional.

f-strings are the recommended way to create strings mixing literals and variable values. For example, node_str = node + "/{child}" would be written node_str = f"{node}/{{child}}".

Single letter variables should be avoided in general. k is used in different places with different meanings; in the first place it should probably be child.

strftime("%Y-%m-%d %H:%M:%S.%f")[:-7] can be simplified to strftime("%Y-%m-%d %H:%M:%S").

The if v is not None filter should probably be in the search_node function. That way you won't have to do the whole filling a variable, copying out the non-None values and replacing the variable rigmarole.

I would probably replace node_str with something like child_xpath, and children with something like child_element_names, for clarity.


The effect of merging the dicts, without actually creating a new dict, can be done using collections.ChainMap.

Rather than clearing info_dict, updating it from filtered and returning it, just return filtered.

from collections import ChainMap

# Merge all dictionaries
# a key gets it's value from earlier dicts
info_dict = ChainMap(misc_info, samp_info, pp_info, model_info, case_info)

# Drop any keys with None values to filter the table
filtered = {k: v for k, v in info_dict.items() if v is not None}
return filtered

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.