如何创建一个带有不可变变量的 python 字典，这些变量正在改变？

Question

I would like to have a dictionary like this one:

primary = "#efefef"

style = {
    "H2": {
        "text-align": "center",
        "color": primary
    }
}

or even better connected dictionaries like these:

colors = {
    "primary" : "#efefef"
}

styles = {
    "H2": {
        "text-align": "center",
        "color": colors["primary"]
    }
}

How can I connect the style dict with the variable primary or the dict colors, so that the dict style changes if primary or colors get new values?

Answer 1

You can use collections.ChainMap to create a mapping that relies on other dicts. This works if the relevant keys are the same, so you can use it to solve your first case:

from collections import ChainMap

primary = {"color": "#efefef"}
style = {
    "H2": ChainMap({"text-align": "center"}, primary),
}

print(style)
primary["color"] = "#000000"
print(style)

Which gives the following output:

{'H2': ChainMap({'text-align': 'center'}, {'color': '#efefef'})}
{'H2': ChainMap({'text-align': 'center'}, {'color': '#000000'})}

---

The second case can be solved with custom dict classes to defer the __getitem__ call:

from collections import UserDict
    

class LazyDict(UserDict):
    def __getitem__(self, key):
        return LazyItem(self.data, key)


class LazyItem:
    def __init__(self, mapping, key):
        self.mapping = mapping
        self.key = key

    @property
    def value(self):
        return self.mapping[self.key]


class EagerDict(dict):
    def __getitem__(self, key):
        obj = super().__getitem__(key)
        if isinstance(obj, LazyItem):
            return obj.value
        return obj


colors = LazyDict({
    "primary" : "#efefef",
})
styles = {
    "H2": EagerDict({
        "text-align": "center",
        "color": colors["primary"],
    }),
}

print(styles["H2"]["color"])
colors["primary"] = "#000000"
print(styles["H2"]["color"])