Add convenience constructors for block encodings (#1243)

* Add convenience constructors for block encodings

* Fix import of `Self`

* Add docstrings

qualtran/bloqs/block_encoding/linear_combination.py

@@ -17,6 +17,7 @@
 
 import numpy as np
 from attrs import evolve, field, frozen, validators
+from typing_extensions import Self
 
 from qualtran import (
     bloq_example,
@@ -106,6 +107,11 @@ def __attrs_post_init__(self):
                 "If given, select oracle must have block encoding `system` register as target."
             )
 
+    @classmethod
+    def of_terms(cls, *terms: Tuple[float, BlockEncoding], lambd_bits: SymbolicInt = 1) -> Self:
+        """Construct a `LinearCombination` from pairs of (coefficient, block encoding)."""
+        return cls(tuple(t[1] for t in terms), tuple(t[0] for t in terms), lambd_bits)
+
     @cached_property
     def signed_block_encodings(self):
         """Appropriately negated constituent block encodings."""

qualtran/bloqs/block_encoding/product.py

@@ -18,6 +18,7 @@
 import cirq
 from attrs import evolve, field, frozen, validators
 from numpy.typing import NDArray
+from typing_extensions import Self
 
 from qualtran import (
     bloq_example,
@@ -87,6 +88,11 @@ def __attrs_post_init__(self):
         if not all(u.system_bitsize == self.system_bitsize for u in self.block_encodings):
             raise ValueError("All block encodings must have the same system size.")
 
+    @classmethod
+    def of(cls, *block_encodings: BlockEncoding) -> Self:
+        """Construct a `Product` from block encodings."""
+        return cls(block_encodings)
+
     @cached_property
     def signature(self) -> Signature:
         return Signature.build_from_dtypes(

qualtran/bloqs/block_encoding/tensor_product.py

@@ -17,6 +17,7 @@
 from typing import Dict, Set, Tuple
 
 from attrs import evolve, field, frozen, validators
+from typing_extensions import Self
 
 from qualtran import (
     bloq_example,
@@ -64,6 +65,11 @@ class TensorProduct(BlockEncoding):
         converter=lambda x: x if isinstance(x, tuple) else tuple(x), validator=validators.min_len(1)
     )
 
+    @classmethod
+    def of(cls, *block_encodings: BlockEncoding) -> Self:
+        """Construct a `TensorProduct` from block encodings."""
+        return cls(block_encodings)
+
     @cached_property
     def signature(self) -> Signature:
         return Signature.build_from_dtypes(