How to read keyboard-input?
I would like to read data from the keyboard in python
I try this:
nb = input('Choose a number')
print ('Number%s \n' % (nb))
But it doesn't work, neither with eclipse nor in the terminal, it's always stop of the question. I can type a number but after nothing happen.
Do you know why?
Use
input('Enter your input:')
if you use Python 3.
And if you want to have a numeric value, just convert it:
try:
mode = int(input('Input:'))
except ValueError:
print("Not a number")
If you use Python 2, you need to use raw_input instead of input.
It seems that you are mixing different Pythons here (Python 2.x vs. Python 3.x)... This is basically correct:
nb = input('Choose a number: ')
The problem is that it is only supported in Python 3. As @sharpner answered, for older versions of Python (2.x), you have to use the function raw_input:
nb = raw_input('Choose a number: ')
If you want to convert that to a number, then you should try:
number = int(nb)
... though you need to take into account that this can raise an exception:
try:
number = int(nb)
except ValueError:
print("Invalid number")
And if you want to print the number using formatting, in Python 3 str.format() is recommended:
print("Number: {0}\n".format(number))
Instead of:
print('Number %s \n' % (nb))
But both options (str.format() and %) do work in both Python 2.7 and Python 3.
Non-blocking, multi-threaded example:
As blocking on keyboard input (since the input() function blocks) is frequently not what we want to do (we'd frequently like to keep doing other stuff), here's a very-stripped-down multi-threaded example to demonstrate how to keep running your main application while still reading in keyboard inputs whenever they arrive. I use this technique in my eRCaGuy_PyTerm serial terminal program here (search the code for input()).
This works by creating one thread to run in the background, continually calling input() and then passing any data it receives to a queue.
In this way, your main thread is left to do anything it wants, receiving the keyboard input data from the first thread whenever there is something in the queue.
1. Bare Python 3 code example (no comments):
import threading
import queue
import time
def read_kbd_input(inputQueue):
print('Ready for keyboard input:')
while (True):
input_str = input()
inputQueue.put(input_str)
def main():
EXIT_COMMAND = "exit"
inputQueue = queue.Queue()
inputThread = threading.Thread(target=read_kbd_input, args=(inputQueue,), daemon=True)
inputThread.start()
while (True):
if (inputQueue.qsize() > 0):
input_str = inputQueue.get()
print("input_str = {}".format(input_str))
if (input_str == EXIT_COMMAND):
print("Exiting serial terminal.")
break
# Insert your code here to do whatever you want with the input_str.
# The rest of your program goes here.
time.sleep(0.01)
print("End.")
if (__name__ == '__main__'):
main()
2. Same Python 3 code as above, but with extensive explanatory comments:
"""
read_keyboard_input.py
Gabriel Staples
www.ElectricRCAircraftGuy.com
14 Nov. 2018
References:
- https://pyserial.readthedocs.io/en/latest/pyserial_api.html
- *****https://www.tutorialspoint.com/python/python_multithreading.htm
- *****https://en.wikibooks.org/wiki/Python_Programming/Threading
- https://stackoverflow.com/questions/1607612/python-how-do-i-make-a-subclass-from-a-superclass
- https://docs.python.org/3/library/queue.html
- https://docs.python.org/3.7/library/threading.html
To install PySerial: `sudo python3 -m pip install pyserial`
To run this program: `python3 this_filename.py`
"""
import threading
import queue
import time
def read_kbd_input(inputQueue):
print('Ready for keyboard input:')
while (True):
# Receive keyboard input from user.
input_str = input()
# Enqueue this input string.
# Note: Lock not required here since we are only calling a single Queue method, not a sequence of them
# which would otherwise need to be treated as one atomic operation.
inputQueue.put(input_str)
def main():
EXIT_COMMAND = "exit" # Command to exit this program
# The following threading lock is required only if you need to enforce atomic access to a chunk of multiple queue
# method calls in a row. Use this if you have such a need, as follows:
# 1. Pass queueLock as an input parameter to whichever function requires it.
# 2. Call queueLock.acquire() to obtain the lock.
# 3. Do your series of queue calls which need to be treated as one big atomic operation, such as calling
# inputQueue.qsize(), followed by inputQueue.put(), for example.
# 4. Call queueLock.release() to release the lock.
# queueLock = threading.Lock()
#Keyboard input queue to pass data from the thread reading the keyboard inputs to the main thread.
inputQueue = queue.Queue()
# Create & start a thread to read keyboard inputs.
# Set daemon to True to auto-kill this thread when all other non-daemonic threads are exited. This is desired since
# this thread has no cleanup to do, which would otherwise require a more graceful approach to clean up then exit.
inputThread = threading.Thread(target=read_kbd_input, args=(inputQueue,), daemon=True)
inputThread.start()
# Main loop
while (True):
# Read keyboard inputs
# Note: if this queue were being read in multiple places we would need to use the queueLock above to ensure
# multi-method-call atomic access. Since this is the only place we are removing from the queue, however, in this
# example program, no locks are required.
if (inputQueue.qsize() > 0):
input_str = inputQueue.get()
print("input_str = {}".format(input_str))
if (input_str == EXIT_COMMAND):
print("Exiting serial terminal.")
break # exit the while loop
# Insert your code here to do whatever you want with the input_str.
# The rest of your program goes here.
# Sleep for a short time to prevent this thread from sucking up all of your CPU resources on your PC.
time.sleep(0.01)
print("End.")
# If you run this Python file directly (ex: via `python3 this_filename.py`), do the following:
if (__name__ == '__main__'):
main()
Sample output:
$ python3 read_keyboard_input.py
Ready for keyboard input:
hey
input_str = hey
hello
input_str = hello
7000
input_str = 7000
exit
input_str = exit
Exiting serial terminal.
End.
The Python Queue library is thread-safe:
Note that Queue.put() and Queue.get() and other Queue class methods are all thread-safe! (This is unlike queues and other containers in the standard template library in C++!) Since the Python Queue class and its methods are thread-safe, that means they implement all the internal locking semantics required for inter-thread operations, so each function call in the queue class can be considered as a single, atomic operation. See the notes at the top of the documentation: https://docs.python.org/3/library/queue.html (emphasis added):
The queue module implements multi-producer, multi-consumer queues. It is especially useful in threaded programming when information must be exchanged safely between multiple threads. The Queue class in this module implements all the required locking semantics.
References:
- https://pyserial.readthedocs.io/en/latest/pyserial_api.html
- *****https://www.tutorialspoint.com/python/python_multithreading.htm
- *****https://en.wikibooks.org/wiki/Python_Programming/Threading
- Python: How do I make a subclass from a superclass?
- https://docs.python.org/3/library/queue.html
- https://docs.python.org/3.7/library/threading.html
- [My repo where I use the techniques and code presented above] https://github.com/ElectricRCAircraftGuy/eRCaGuy_PyTerm
Related/Cross-Linked:
- [my answer] PySerial non-blocking read loop