### Discussed in https://github.com/apache/airflow/discussions/36076

<div type='discussions-op-text'>

<sup>Originally posted by **timc** December  5, 2023</sup>
### Apache Airflow version

2.7.3

### What happened

When creating a virtual env task and passing in a requirements file like this:

`@task.virtualenv(
            use_dill=True,
            system_site_packages=False,
            requirements='requirements.txt')`

The result is that the contents of the requirements file using to populate the venv is

requirements.txt

Which is wrong. And you get this:

[2023-12-05, 12:33:06 UTC] {{process_utils.py:181}} INFO - Executing cmd: python3 /usr/local/***/.local/lib/python3.10/site-packages/virtualenv /tmp/venv3cdlqjlq
[2023-12-05, 12:33:06 UTC] {{process_utils.py:185}} INFO - Output:
[2023-12-05, 12:33:07 UTC] {{process_utils.py:189}} INFO - created virtual environment CPython3.10.9.final.0-64 in 397ms
[2023-12-05, 12:33:07 UTC] {{process_utils.py:189}} INFO -   creator CPython3Posix(dest=/tmp/venv3cdlqjlq, clear=False, no_vcs_ignore=False, global=False)
[2023-12-05, 12:33:07 UTC] {{process_utils.py:189}} INFO -   seeder FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=/usr/local/***/.local/share/virtualenv)
[2023-12-05, 12:33:07 UTC] {{process_utils.py:189}} INFO -     added seed packages: pip==23.3.1, setuptools==69.0.2, wheel==0.42.0
[2023-12-05, 12:33:07 UTC] {{process_utils.py:189}} INFO -   activators BashActivator,CShellActivator,FishActivator,NushellActivator,PowerShellActivator,PythonActivator
[2023-12-05, 12:33:07 UTC] {{process_utils.py:181}} INFO - Executing cmd: /tmp/venv3cdlqjlq/bin/pip install -r /tmp/venv3cdlqjlq/requirements.txt
[2023-12-05, 12:33:07 UTC] {{process_utils.py:185}} INFO - Output:
[2023-12-05, 12:33:09 UTC] {{process_utils.py:189}} INFO - ERROR: Could not find a version that satisfies the requirement requirements.txt (from versions: none)
[2023-12-05, 12:33:09 UTC] {{process_utils.py:189}} INFO - HINT: You are attempting to install a package literally named "requirements.txt" (which cannot exist). Consider using the '-r' flag to install the packages listed in requirements.txt
[2023-12-05, 12:33:09 UTC] {{process_utils.py:189}} INFO - ERROR: No matching distribution found for requirements.txt
[2023-12-05, 12:33:09 UTC] {{taskinstance.py:1824}} ERROR - Task failed with exception

The issue appears to be that the requirements parameter is added to a list on construction of the operator so the templating never happens.

### What you think should happen instead

The provided requirements file should be used in the pip command to set up the venv.

### How to reproduce

Create a dag:

```
from datetime import datetime
from airflow.decorators import dag, task

 @dag(schedule_interval=None, start_date=datetime(2021, 1, 1), catchup=False, tags=['example']) 
def virtualenv_task():
    @task.virtualenv(
        use_dill=True,
        system_site_packages=False,
        requirements='requirements.txt',
    )
    def extract():
        import pandas 
        x = pandas.DataFrame()

    extract()

dag = virtualenv_task()
```
And a requirements.txt file

```
pandas
```

Run AirFlow

### Operating System

Ubuntu 23.04

### Versions of Apache Airflow Providers

apache-airflow-providers-amazon==8.2.0
apache-airflow-providers-celery==3.2.1
apache-airflow-providers-common-sql==1.5.2
apache-airflow-providers-ftp==3.4.2
apache-airflow-providers-http==4.4.2
apache-airflow-providers-imap==3.2.2
apache-airflow-providers-postgres==5.5.1
apache-airflow-providers-sqlite==3.4.2

### Deployment

Docker-Compose

### Deployment details

_No response_

### Anything else

Everytime.

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)
</div>
"""Return mapped nodes that depend on the current task the expansion.
        For now, this walks the entire DAG to find mapped nodes that has this
        current task as an upstream. We cannot use ``downstream_list`` since it
        only contains operators, not task groups. In the future, we should
        provide a way to record an DAG node's all downstream nodes instead.
        """
        return (
            downstream
            for downstream in self._iter_all_mapped_downstreams()
            if any(p.node_id == self.node_id for p in downstream.iter_mapped_dependencies())
        )
    def iter_mapped_task_groups(self) -> Iterator[MappedTaskGroup]: