In `pexpect`, how to avoid `sendline` argument being part of `before` in the following?

```python
import pexpect

analyzer = pexpect.spawn('adb shell')
PROMPT = r'FP4:/ \$ '
analyzer.expect(PROMPT)

analyzer.sendline('echo -n "a"')
analyzer.expect(PROMPT)
print(f'{analyzer.before=}')
```
```
analyzer.before=b'echo -n "a"\r\r\na'
```
Aug 19, 12:10pm

In pexpect, the before attribute contains all the output received before the expected pattern (e.g., PROMPT) is matched. When you use sendline, the command you send is echoed back by the terminal (e.g., adb shell), which is why it appears in before.

How to Avoid sendline Argument in before

To exclude the command itself from before, you have a few options:

1. Use send + Manual Newline

Instead of sendline, use send and manually add a newline (\r or \n). This avoids pexpect automatically echoing the command in some terminal emulators.

python
analyzer.send('echo -n "a"\r')  # Manually add newline
analyzer.expect(PROMPT)
print(f'{analyzer.before=}')

Why?

  • sendline = send + \r\n (newline).
  • Some terminals echo the command back, which pexpect captures in before.
  • Using send + explicit newline may bypass this behavior.

2. Use logfile_read to Filter Output

If the terminal always echoes commands, you can suppress it by:

  • Redirecting output to a log file.
  • Parsing the output to exclude the command.
python
import io
log = io.StringIO()
analyzer.logfile_read = log  # Capture output to log

analyzer.sendline('echo -n "a"')
analyzer.expect(PROMPT)
output = log.getvalue()
log.close()
print(f'Output: {output}')

Why?

  • This captures only the terminal's response, not the command.

3. Use expect with a Regex to Skip the Command

If the command is always echoed, modify your expect pattern to skip it:

python
analyzer.sendline('echo -n "a"')
analyzer.expect(r'echo -n "a"\r\r\na.*' + PROMPT)  # Skip the command
print(f'{analyzer.after=}')  # Use `after` instead of `before`

Why?

  • after contains the text after the matched pattern.
  • This avoids including the command in before.

4. Disable Terminal Echo (If Possible)

If you control the terminal (e.g., adb shell), disable command echoing:

python
analyzer.sendline('stty -echo')  # Disable echo (Linux/Unix)
analyzer.sendline('echo -n "a"')
analyzer.expect(PROMPT)
print(f'{analyzer.before=}')
analyzer.sendline('stty echo')   # Re-enable echo

Why?

  • Prevents the terminal from echoing commands back.

Best Approach for Your Case

Since adb shell echoes commands by default, Option 3 (Regex in expect) or Option 2 (Logfile) are the most reliable. Example:

python
analyzer.sendline('echo -n "a"')
analyzer.expect(r'echo -n "a"\r\r\na.*' + PROMPT)
print(f'{analyzer.after=}')  # Output: b'a'

This ensures before does not include the command.

Aug 19, 12:10pm

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