python: Raise ValueError for non-existant tx hashes

If the device asks for a tx_hash which is not present in prev_txes, raise a ValueError with some more detailed messaging about the missing hash rather than the default dictionary lookup failure of KeyError.
trezor · Feb 4, 2021 · be46644 · be46644
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diff --git a/python/CHANGELOG.md b/python/CHANGELOG.md
@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Enabled session management via `EndSession`  [#1227]
 - Support for temporary or permanent `safety-checks` setting
 - Support for Output Descriptors export [#1363]
+- Raises `ValueError` when the txid for an input is not present in `prev_txes` during `btc.sign_tx` [#1442]
 
 ### Changed
 

diff --git a/python/src/trezorlib/btc.py b/python/src/trezorlib/btc.py
@@ -279,6 +279,8 @@ def copy_tx_meta(tx: messages.TransactionType) -> messages.TransactionType:
 
         # Device asked for one more information, let's process it.
         if res.details.tx_hash is not None:
+            if res.details.tx_hash not in prev_txes:
+                raise ValueError(f"Previous transaction {res.details.tx_hash.hex()} not available")
             current_tx = prev_txes[res.details.tx_hash]
         else:
             current_tx = this_tx