[Python-ideas] Syntax to conditionally define a field in a dict

[Joshua Marshall]

Hello all,

I have a use case where I need to send a `dict` to a module as an
argument.  Inside of this, it has a multi-level structure, but each field I
need to set may only be set to a single value.  Fields must be valid,
non-empty strings.  It looks a lot like the following in my code:

```
def my_func(val_1, val_2):
    return {
        "field_1": val_1,
        "next_depth": {
            "field_2": val_2
        }
    }
```

What I want to do is:
```
def my_func(val_1, val_2):
    return {
        "field_1": val_1 if val_1,
        "next_depth": {
            "field_2": val_2 if val_2
        }
    }
```

Or:
```
def my_func(val_1, val_2):
    return {
        if val_1 : "field_1": val_1,
        "next_depth": {
            if val_2: "field_2": val_2
        }
    }
```

Where each conditional in this context functions as:
```
if value:
    d["your_key"] = value
```
for each conditionally added and set key.

>From the slack channel #learning_python, there are a number of more general
points which need to be handled.  The more core syntax, which should be
valid throughout the language, would be to have statements like `x = y if
cond` and `x[y if cond]`.  The first of these intuitively reorganizes to
`if cond: x = y`, but the second is not as clear, with a likely equivalent
of `if cond: x[y] else raise Exception`.

Thanks to Tom Forbes and Jim Kelly for helping critique the idea thus far.

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