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<@ULVA73B9P>
@flow(name="NETS-Extraction",flow_run_name='txn:{txn_path[-16:-8]}-prf:{profiles_path[-16:-8]}',log_prints=True)
def run(txn_path:str,profiles_path:str):

flow_run_name = flow.flow_run_name.format(**parameters)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: string indices must be integers, not 'str'

:white_check_mark: thought for 90.3 seconds

`flow_run_name` string templates only support *simple parameter formatting* like `{txn_path}` — not Python expressions like slicing.

So this:

```python
@flow(
    name="NETS-Extraction",
    flow_run_name="txn:{txn_path[-16:-8]}-prf:{profiles_path[-16:-8]}",
    log_prints=True,
)
def run(txn_path: str, profiles_path: str):
    ...
```

fails because Prefect treats `flow_run_name` as:

```python
flow_run_name.format(**parameters)
```

and Python `str.format()` cannot evaluate `[-16:-8]` inside `{...}`.

That’s why you get:

```python
TypeError: string indices must be integers, not 'str'
```

*What to do instead*

*Option 1: use a callable for `flow_run_name`*  
This is the cleanest fix for your case.

```python
from prefect import flow
from prefect.context import FlowRunContext

def make_flow_run_name():
    ctx = FlowRunContext.get()
    params = ctx.parameters if ctx else {}

    txn_path = params.get("txn_path", "")
    profiles_path = params.get("profiles_path", "")

    txn_part = txn_path[-16:-8] if len(txn_path) &gt;= 16 else txn_path
    profiles_part = profiles_path[-16:-8] if len(profiles_path) &gt;= 16 else profiles_path

    return f"txn:{txn_part}-prf:{profiles_part}"

@flow(
    name="NETS-Extraction",
    flow_run_name=make_flow_run_name,
    log_prints=True,
)
def run(txn_path: str, profiles_path: str):
    print("running")
```

*Option 2: pass precomputed parameters and use simple placeholders*

```python
from prefect import flow

@flow(
    name="NETS-Extraction",
    flow_run_name="txn:{txn_part}-prf:{profiles_part}",
    log_prints=True,
)
def run(txn_path: str, profiles_path: str, txn_part: str, profiles_part: str):
    print("running")

run(
    txn_path="some/long/path/20240101_file.csv",
    profiles_path="some/other/path/20240102_file.csv",
    txn_part="20240101",
    profiles_part="20240102",
)
```

*Important detail*

For flows, a callable `flow_run_name` is called with *no arguments*, so this will *not* work:

```python
def make_flow_run_name(parameters):
    ...
```

Instead, use `FlowRunContext.get()` as shown above.

*In short:*  
- `{txn_path}` works
- `{txn_path[-16:-8]}` does *not* work
- use a callable if you need slicing/logic

Relevant source-backed behavior:
- Prefect 3.x resolves string `flow_run_name` via `str.format(**parameters)`
- callable `flow_run_name` is supported for flows, but called without parameters

If you want, I can also rewrite your exact decorator/code into a drop-in fixed version.