diff --git a/python/src/trezorlib/btc.py b/python/src/trezorlib/btc.py
index 7dcbaa238..b15241693 100644
--- a/python/src/trezorlib/btc.py
+++ b/python/src/trezorlib/btc.py
@@ -15,40 +15,45 @@
 # If not, see <https://www.gnu.org/licenses/lgpl-3.0.html>.
 
 from decimal import Decimal
+from typing import TYPE_CHECKING, Any, Dict, Sequence, Tuple
 
 from . import exceptions, messages
 from .tools import expect, normalize_nfc, session
 
+if TYPE_CHECKING:
+    from .client import TrezorClient
+
 
 def from_json(json_dict):
     def make_input(vin):
-        i = messages.TxInputType()
         if "coinbase" in vin:
-            i.prev_hash = b"\0" * 32
-            i.prev_index = 0xFFFFFFFF  # signed int -1
-            i.script_sig = bytes.fromhex(vin["coinbase"])
-            i.sequence = vin["sequence"]
+            return messages.TxInputType(
+                prev_hash=b"\0" * 32,
+                prev_index=0xFFFFFFFF,  # signed int -1
+                script_sig=bytes.fromhex(vin["coinbase"]),
+                sequence=vin["sequence"],
+            )
 
         else:
-            i.prev_hash = bytes.fromhex(vin["txid"])
-            i.prev_index = vin["vout"]
-            i.script_sig = bytes.fromhex(vin["scriptSig"]["hex"])
-            i.sequence = vin["sequence"]
-
-        return i
+            return messages.TxInputType(
+                prev_hash=bytes.fromhex(vin["txid"]),
+                prev_index=vin["vout"],
+                script_sig=bytes.fromhex(vin["scriptSig"]["hex"]),
+                sequence=vin["sequence"],
+            )
 
     def make_bin_output(vout):
-        o = messages.TxOutputBinType()
-        o.amount = int(Decimal(vout["value"]) * (10 ** 8))
-        o.script_pubkey = bytes.fromhex(vout["scriptPubKey"]["hex"])
-        return o
+        return messages.TxOutputBinType(
+            amount=int(Decimal(vout["value"]) * (10 ** 8)),
+            script_pubkey=bytes.fromhex(vout["scriptPubKey"]["hex"]),
+        )
 
-    t = messages.TransactionType()
-    t.version = json_dict["version"]
-    t.lock_time = json_dict.get("locktime")
-    t.inputs = [make_input(vin) for vin in json_dict["vin"]]
-    t.bin_outputs = [make_bin_output(vout) for vout in json_dict["vout"]]
-    return t
+    return messages.TransactionType(
+        version=json_dict["version"],
+        lock_time=json_dict.get("locktime"),
+        inputs=[make_input(vin) for vin in json_dict["vin"]],
+        bin_outputs=[make_bin_output(vout) for vout in json_dict["vout"]],
+    )
 
 
 @expect(messages.PublicKey)
@@ -173,24 +178,31 @@ def verify_message(client, coin_name, address, signature, message):
 
 @session
 def sign_tx(
-    client,
-    coin_name,
-    inputs,
-    outputs,
-    details=None,
-    prev_txes=None,
-    preauthorized=False,
-):
-    this_tx = messages.TransactionType(inputs=inputs, outputs=outputs)
-
-    if details is None:
-        signtx = messages.SignTx()
-    else:
-        signtx = details
-
-    signtx.coin_name = coin_name
-    signtx.inputs_count = len(inputs)
-    signtx.outputs_count = len(outputs)
+    client: "TrezorClient",
+    coin_name: str,
+    inputs: Sequence[messages.TxInputType],
+    outputs: Sequence[messages.TxOutputType],
+    prev_txes: Dict[bytes, messages.TransactionType],
+    preauthorized: bool = False,
+    **kwargs: Any,
+) -> Tuple[Sequence[bytes], bytes]:
+    """Sign a Bitcoin-like transaction.
+
+    Returns a list of signatures (one for each provided input) and the
+    network-serialized transaction.
+
+    In addition to the required arguments, it is possible to specify additional
+    transaction properties (version, lock time, expiry...). Each additional argument
+    must correspond to a field in the `SignTx` data type. Note that some fields
+    (`inputs_count`, `outputs_count`, `coin_name`) will be inferred from the arguments
+    and cannot be overriden by kwargs.
+    """
+    signtx = messages.SignTx(
+        coin_name=coin_name, inputs_count=len(inputs), outputs_count=len(outputs),
+    )
+    for name, value in kwargs.items():
+        if hasattr(signtx, name):
+            setattr(signtx, name, value)
 
     if preauthorized:
         res = client.call(messages.DoPreauthorized())
@@ -203,7 +215,7 @@ def sign_tx(
     signatures = [None] * len(inputs)
     serialized_tx = b""
 
-    def copy_tx_meta(tx):
+    def copy_tx_meta(tx: messages.TransactionType) -> messages.TransactionType:
         tx_copy = messages.TransactionType(**tx)
         # clear fields
         tx_copy.inputs_cnt = len(tx.inputs)
@@ -215,6 +227,15 @@ def sign_tx(
         tx_copy.extra_data = None
         return tx_copy
 
+    this_tx = messages.TransactionType(
+        inputs=inputs,
+        outputs=outputs,
+        inputs_cnt=len(inputs),
+        outputs_cnt=len(outputs),
+        # pick either kw-provided or default value from the SignTx request
+        version=signtx.version,
+    )
+
     R = messages.RequestType
     while isinstance(res, messages.TxRequest):
         # If there's some part of signed transaction, let's add it