12.11 The msvcrt Module

(Windows/DOS only) The msvcrt module gives you access to a number of functions in the Microsoft Visual C/C++ Runtime Library (MSVCRT).

Example 12-13 demonstrates the getch function reading a single keypress from the console.

Example 12-13. Using the msvcrt Module to Get Key Presses

File: msvcrt-example-1.py

import msvcrt

print "press 'escape' to quit..."

while 1:
    char = msvcrt.getch()
    if char == chr(27):
        break
    print char,
    if char == chr(13):
        print

press 'escape' to quit...
h e l l o

The kbhit function returns true if a key has been pressed (which means that getch won't block), as shown in Example 12-14.

Example 12-14. Using the msvcrt Module to Poll the Keyboard

File: msvcrt-example-2.py

import msvcrt
import time

print "press SPACE to enter the serial number"

while not msvcrt.kbhit() or msvcrt.getch() != " ":
    # do something else
    print ".",
    time.sleep(0.1)

print

# clear the keyboard buffer
while msvcrt.kbhit():
    msvcrt.getch()

serial = raw_input("enter your serial number: ")

print "serial number is", serial

press SPACE to enter the serial number
. . . . . . . . . . . . . . . . . . . . . . . .
enter your serial number: 10
serial number is 10

The locking function in Example 12-15 can be used to implement cross-process file locking under Windows.

Example 12-15. Using the msvcrt Module for File Locking

File: msvcrt-example-3.py

import msvcrt
import os

LK_UNLCK = 0 # unlock the file region
LK_LOCK = 1 # lock the file region
LK_NBLCK = 2 # non-blocking lock
LK_RLCK = 3 # lock for writing
LK_NBRLCK = 4 # non-blocking lock for writing

FILE = "counter.txt"

if not os.path.exists(FILE):
    file = open(FILE, "w")
    file.write("0")
    file.close()

for i in range(20):
    file = open(FILE, "r+")
    # look from current position (0) to end of file
    msvcrt.locking(file.fileno(), LK_LOCK, os.path.getsize(FILE))
    counter = int(file.readline()) + 1
    file.seek(0)
    file.write(str(counter))
    file.close() # unlocks the file
    print os.getpid(), "=>", counter
    time.sleep(0.1)

208 => 21
208 => 22
208 => 23
208 => 24
208 => 25
208 => 26