Develop Reference

Python - Checking if there is an input from user without stopping the loop

I have a small script where I have a continuous loop. The loop runs and I expect that the user will give an input whenever he wants. I want to check if the user wrote something in the console, if yes, I will do something extra, if not, the loop continues.
The problem is that when I call the input() function, the program waits for user input.
The code that I will use here will be just a simple version of my code to show better what is my problem.
i=0
while True:
i+=1
if 'user wrote a number':
i+= 'user's input'
The objective is to not stop the loop if the user do not input anything. I believe this is a simple thing but I didn't find an answer for this problem.
Thank you for your time!

You can execute the background task (the while True) on a separate thread, and let the main thread handle the input.
import time
import threading
import sys
def background():
while True:
time.sleep(3)
print('background task')
def handling_input(inp):
print('Got {}'.format(inp))
t = threading.Thread(target=background)
t.daemon = True
t.start()
while True:
inp = input()
handling_input(inp)
if inp == 'q':
print('quitting')
sys.exit()
Sample execution (lines starting with >> are my inputs):
background task
>> a
Got a
>> b
Got b
background task
>> cc
Got cc
background task
background task
>> q
Got q
quitting
Process finished with exit code 0
The only caveat is if the user takes longer than 3 seconds to type (or whatever the value of time.sleep in background is) the input value will be truncated.

I'm don't think you can do that in one single process input(), because Python is a synchronous programming languaje,the execution will be stoped until input() receives a value.
As a final solution I'd recommend you to try implement your functions with parallel processing so that both 'functions' (input and loop) can get executed at the same time, then when the input function gets it's results, it sends the result to the other process (the loop) and finish the execution.

Not executing the code outside of the while loop after the break statement

I'm trying to exit an infinite loop but I'm stuck in it. I've tried using the break statement in different ways but the result is always the same: the program terminates after exiting the loop and it doesn't perform the rest of the code. What should I change in the code to solve this problem?
moylist = []
import copy
while True:
thing = input()
if thing == '':
break
moylist.append(thing)
moylist.insert(-2, 'and')
moynovlist = copy.deepcopy(moylist)
moynovlist1 = moynovlist[-2:] # cutting the end
perv = str(moynovlist1)
perv1 = perv[:4] + perv[4:] #without the comma (and ...)

The code is running fine! The reason you think it is exiting the whole program instead of just the while loop is because you don't have any print statements. Simply add, print(perv1) at the end of your program and you'll see that perv1 changes, meaning the loop was exited properly.

Python does't run some lines of code as it should

can someone help me with this code? It's use is to make timed input for 2 seconds and it needs to print the last line even if the user did or didn't type something in the console (for that time). When I don't type in anything, after printing "Too Slow" it asks me for the same input, so I added IF, but it doesn't really notice it. I hope that someone can help me. Thanks!
import time
from threading import Thread
i = 0
answer = None
def check():
time.sleep(2)
if answer != None:
return
print("Too Slow") #prints this if nothing is typed in (for 2 seconds)
if i == 0:
i = 1
Thread(target = check).start()
answer = input("Input something: ") #program doesn't even notice IF and asks me the second time for input
print("This should be printed instantly after printing Too Slow (when user doesn't input anything)")

The return in the check function only causes the thread to be completed, but the call to input is independent of that thread and thus not interrupted when the check function completes. See this answer for an alternative solution using the signal module.

taking multiline input with sys.stdin

I have the following function:
def getInput():
# define buffer (list of lines)
buffer = []
run = True
while run:
# loop through each line of user input, adding it to buffer
for line in sys.stdin.readlines():
if line == 'quit\n':
run = False
else:
buffer.append(line.replace('\n',''))
# return list of lines
return buffer
which is called in my function takeCommands(), which is called to actually run my program.
However, this doesn't do anything. I'm hoping to add each line to an array, and once a line == 'quit' it stops taking user input. I've tried both for line in sys.stdin.readlines() and for line sys.stdin, but neither of them register any of my input (I'm running it in Windows Command Prompt). Any ideas? Thanks

So, took your code out of the function and ran some tests.
import sys
buffer = []
while run:
line = sys.stdin.readline().rstrip('\n')
if line == 'quit':
run = False
else:
buffer.append(line)
print buffer
Changes:
Removed the 'for' loop
Using 'readline' instead of 'readlines'
strip'd out the '\n' after input, so all processing afterwards is much easier.
Another way:
import sys
buffer = []
while True:
line = sys.stdin.readline().rstrip('\n')
if line == 'quit':
break
else:
buffer.append(line)
print buffer
Takes out the 'run' variable, as it is not really needed.

I'd use itertools.takewhile for this:
import sys
import itertools
print list(itertools.takewhile(lambda x: x.strip() != 'quit', sys.stdin))
Another way to do this would be to use the 2-argument iter form:
print list(iter(raw_input,'quit'))
This has the advantage that raw_input takes care of all of the line-buffering issues and it will strip the newlines for you already -- But it will loop until you run out of memory if the user forgets to add a quit to the script.
Both of these pass the test:
python test.py <<EOF
foo
bar
baz
quit
cat
dog
cow
EOF

There are multiple separate problems with this code:
while run:
# loop through each line of user input, adding it to buffer
for line in sys.stdin.readlines():
if line == 'quit':
run = False
First, you have an inner loop that won't finish until all lines have been processed, even if you type "quit" at some point. Setting run = False doesn't break out of that loop. Instead of quitting as soon as you type "quit", it will keep going until it's looked at all of the lines, and then quit if you typed "quit" at any point.
You can fix this one pretty easily by adding a break after the run = False.
But, with or without that fix, if you didn't type "quit" during that first time through the outer loop, since you've already read all input, there's nothing else to read, so you'll just keep running an empty inner loop over and over forever that you can never exit.
You have a loop that means "read and process all the input". You want to do that exactly once. So, what should the outer loop be? It should not be anyway; the way to do something once is to not use a loop. So, to fix this one, get rid of run and the while run: loop; just use the inner loop.
Then, if you type "quit", line will actually be "quit\n", because readlines does not strip newlines.
You fix this one by either testing for "quit\n", or stripping the lines.
Finally, even if you fix all of these problems, you're still waiting forever before doing anything. readlines returns a list of lines. The only way it can possibly do that is by reading all of the lines that will ever be on stdin. You can't even start looping until you've read all those lines.
When standard input is a file, that happens when the file ends, so it's not too terrible. But when standard input is the Windows command prompt, the command prompt never ends.* So, this takes forever. You don't get to start processing the list of lines, because it takes forever to wait for the list of lines.
The solution is to not use readlines(). Really, there is never a good reason to call readlines() on anything, stdin or not. Anything that readlines works on is already an iterable full of lines, just like the list that readlines would give you, except that it's "lazy": it can give you the lines one at a time, instead of waiting and giving you all of them at once. (And even if you really need the list, just do list(f) instead of f.readlines().)
So, instead of for line in sys.stdin.readlines():, just do for line in sys.stdin: (Or, better, replace the explicit loop completely and use a sequence of iterator transformations, as in mgilson's answer.)
The fixes JBernardo, Wing Tang Wong, etc. proposed are all correct, and necessary. The reason none of them fixed your problems is that if you have 4 bugs and fix 1, your code still doesn't work. That's exactly why "doesn't work" isn't a useful measure of anything in programming, and you have to debug what's actually going wrong to know whether you're making progress.
* I lied a bit about stdin never being finished. If you type a control-Z (you may or may not need to follow it with a return), then stdin is finished. But if your assignment is to make it quit as soon as the user types "quit"< turning in something that only quits when the user types "quit" and then return, control-Z, return again probably won't be considered successful.

Can't kill my python code. What's wrong?

Okay, so I'm writing a very simplistic password cracker in python that brute forces a password with alphanumeric characters. Currently this code only supports 1 character passwords and a password file with a md5 hashed password inside. It will eventually include the option to specify your own character limits (how many characters the cracker tries until it fails). Right now I cannot kill this code when I want it to die. I have included a try and except snippit, however it's not working. What did I do wrong?
Code: http://pastebin.com/MkJGmmDU
import linecache, hashlib
alphaNumeric = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z",1,2,3,4,5,6,7,8,9,0]
class main:
def checker():
try:
while 1:
if hashlib.md5(alphaNumeric[num1]) == passwordHash:
print "Success! Your password is: " + str(alphaNumeric[num1])
break
except KeyboardInterrupt:
print "Keyboard Interrupt."
global num1, passwordHash, fileToCrack, numOfChars
print "What file do you want to crack?"
fileToCrack = raw_input("> ")
print "How many characters do you want to try?"
numOfChars = raw_input("> ")
print "Scanning file..."
passwordHash = linecache.getline(fileToCrack, 1)[0:32]
num1 = 0
checker()
main

The way to allow a KeyboardInterrupt to end your program is to do nothing. They work by depending on nothing catching them in an except block; when an exception bubbles all the way out of a program (or thread), it terminates.
What you have done is to trap the KeyboardInterrupts and handle them by printing a message and then continuing.
As for why the program gets stuck, there is nothing that ever causes num1 to change, so the md5 calculation is the same calculation every time. If you wanted to iterate over the symbols in alphaNumeric, then do that: for symbol in alphaNumeric: # do something with 'symbol'.
Of course, that will still only consider every possible one-character password. You're going to have to try harder than that... :)
I think you're also confused about the use of classes. Python does not require you to wrap everything inside a class. The main at the end of your program does nothing useful; your code runs because it is evaluated when the compiler tries to figure out what a main class is. This is an abuse of syntax. What you want to do is put this code in a main function, and call the function (the same way you call checker currently).

Besides printing, you need to actually exit your program when capturin KeyboardInterrupt, you're only printing a message.

This is what worked for me...
import sys
try:
....code that hangs....
except KeyboardInterrupt:
print "interupt"
sys.exit()

Well, when you use that try and except block, the error is raised when that error occurs. In your case, KeyboardInterrupt is your error here. But when KeyboardInterrupt is activated, nothing happens. This due to having nothing in the except part. You could do this after importing sys:
try:
#Your code#
except KeyboardInterrupt:
print 'Put Text Here'
sys.exit()
sys.exit() is an easy way to safely exit the program. This can be used for making programs with passwords to end the program if the password is wrong or something like that. That should fix the except part. Now to the try part:
If you have break as the end of the try part, nothing is going to happen. Why? Because break only works on loops, most people tend to do it for while loops. Let's make some examples. Here's one:
while 1:
print 'djfgerj'
break
The break statement will stop and end the loop immediately unlike its brother continue, which continues the loop. That's just extra information. Now if you have break in a something like this:
if liners == 0:
break
That's going to depend where that if statement is. If it is in a loop, it is going to stop the loop. If not, nothing is going to happen. I am assuming you made an attempt to exit the function which didn't work. It looks like the program should end, so use sys.exit() like I showed you above. Also, you should group that last piece of code (in the class) into a seperate function. I hope this helps you!