Digests as Arguments

fleche supports passing Digest objects as arguments to functions. When a Digest is passed as a top-level argument (either positional or keyword), fleche automatically expands it to its cached value before calling the decorated function.

This allows for efficient chaining of cached functions where you don’t need to load the full result into memory if you’re just going to pass it to another cached function.

Usage

You can use the convenience wrapper D to mark a string as a digest:

from fleche import fleche, D

@fleche
def func_a(x):
    return x + 1

@fleche
def func_b(y):
    return y * 2

# Get the digest of the result of func_a(5)
print(func_a.digest(5))
# '0e69e4e8496e7caebdefd277a0dfbd65929419a52f6183ace4000d4029add987'

# Pass it to func_b. func_b will receive the value 6, not the digest string.
# You can even use a short digest!
result = func_b(D('0e69e4e8'))
assert result == 12

Behavior

  • Automatic Expansion: Only immediate Digest arguments are expanded.

  • Non-recursive: If you pass a list of digests like [D(a), D(b)], the list will be passed as-is to the function, and the digests inside it will NOT be expanded.

  • Missing Digests: If a Digest is passed but its value cannot be found in the current cache, a KeyError will be raised.

  • Short Digests: If the underlying storage supports short-hand digest expansion, you can pass a short digest (at least 4 characters) to D(), and it will be expanded to the full value.

The D Wrapper

fleche.D(value) → Digest[source]

Convenience wrapper to create a Digest from a value.

If given a non-empty string that is a valid hexadecimal string of at most DIGEST_LENGTH characters, wraps it in a Digest directly (allowing short hex prefixes for use with expand()). For any other value — including strings that are not valid hex digests — computes the digest.

Warning

Passing an arbitrary string to D() will silently compute its digest rather than treating the string as a digest identifier. Only strings that consist entirely of hexadecimal characters (0-9, a-f, A-F) and are no longer than DIGEST_LENGTH characters are used verbatim. If you intend to look up a cached value by its digest string, make sure the string you pass satisfies those constraints; otherwise you will get the digest of the string itself, which is almost certainly not what you want.

Digests passed as arguments to @fleche decorated functions are automatically expanded to their cached values.