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Q&A Listen for key events in a CLI app

First of all, the standard keystroke for interrupting a CLI program would be ctrl + C or whatever the windows equivalent is. Therefore, you will get the most consistent user experience if you use ...

posted 8mo ago by mr Tsjolder‭ · edited 8mo ago by mr Tsjolder‭

Answer

#2: Post edited by

mr Tsjolder‭ · 2023-09-09T13:00:13Z (8 months ago)
add warning about expecting program to exit fast.

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First of all, the standard keystroke for interrupting a CLI program would be `ctrl + C` or whatever the windows equivalent is.
Therefore, you will get the most consistent user experience if you use this standard keystroke for interrupting the program (rather than something custom like space).
One big advantage of using standard keystrokes, is that Python will raise a [`KeyboardInterrupt`](https://docs.python.org/3/library/exceptions.html#KeyboardInterrupt) exception upon detection.
As a result, we just have to write error handling code to stop the program properly.
A Minimal Working Example (MWE) of this code could look as follows:
```python
import time
done = False
while not done:
try:
print("looping")
time.sleep(1)
except KeyboardInterrupt:
done = True
print("done")
```
It might be even (slightly) better to wrap the entire code as follows:
```python
import time
try:
while True:
print("looping")
time.sleep(1)
except KeyboardInterrupt: # not strictly necessary
pass
finally:
print("done")
```

First of all, the standard keystroke for interrupting a CLI program would be `ctrl + C` or whatever the windows equivalent is.
Therefore, you will get the most consistent user experience if you use this standard keystroke for interrupting the program (rather than something custom like space).
One big advantage of using standard keystrokes, is that Python will raise a [`KeyboardInterrupt`](https://docs.python.org/3/library/exceptions.html#KeyboardInterrupt) exception upon detection.
As a result, we just have to write error handling code to stop the program properly.
A Minimal Working Example (MWE) of this code could look as follows:
```python
import time
done = False
while not done:
try:
print("looping")
time.sleep(1)
except KeyboardInterrupt:
done = True
print("done")
```
It might be even (slightly) better to wrap the entire code as follows:
```python
import time
try:
while True:
print("looping")
time.sleep(1)
except KeyboardInterrupt: # not strictly necessary
pass
finally:
print("done")
```
---
**Addendum:** as indicated in the linked documentation (and mentioned in the comments), it would be important that the `finish_up` function has minimal runtime and allows to exit the program as quickly as possible.
If the code in `finish_up` is lengthy, it might be useful/necessary to split up the code in a part that makes sure the program leaves a consistent state (if possible without too much overhead) and a (possibly slow) post-processing part that would appear inside of the `try` block. E.g.
```python
import time
try:
while True:
print("looping")
time.sleep(1)
post_processing()
except KeyboardInterrupt: # not strictly necessary
pass
finally:
print("finishing")
clean_up()
```

#1: Initial revision by

mr Tsjolder‭ · 2023-09-08T07:03:00Z (8 months ago)

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First of all, the standard keystroke for interrupting a CLI program would be `ctrl + C` or whatever the windows equivalent is.
Therefore, you will get the most consistent user experience if you use this standard keystroke for interrupting the program (rather than something custom like space).

One big advantage of using standard keystrokes, is that Python will raise a [`KeyboardInterrupt`](https://docs.python.org/3/library/exceptions.html#KeyboardInterrupt) exception upon detection.
As a result, we just have to write error handling code to stop the program properly.

A Minimal Working Example (MWE) of this code could look as follows:
```python
import time

done = False
while not done:
    try:
        print("looping")
        time.sleep(1)
    except KeyboardInterrupt:
        done = True

print("done")
```

It might be even (slightly) better to wrap the entire code as follows:
```python
import time

try:
    while True:
        print("looping")
        time.sleep(1)
except KeyboardInterrupt:  # not strictly necessary
    pass
finally:
    print("done")
```

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