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RLock (re-entrant lock) is a type of lock that allows the holder of the lock to acquire it multiple times without causing a deadlock. This can be useful in situations where a thread needs to acquire the same lock multiple times while performing some operation.

Note: If a code can be safely called again, it is re-entrant. In other words, re-entrant code can be called multiple times and still function correctly even if called by different threads. As a result, the re-entrant section of code typically uses local variables only in such a way that each call to the code receives its own unique copy of data.

Re-entrant code:

def f(i): return i + 2 
def h(i): return f(i) + 2;

Normal Lock objects can only be obtained once, even by the same thread. If a lock is accessed by more than one function in the same call chain, this can have unintended consequences:

Example:

import threading

lock = threading.Lock()

print 'First try :', lock.acquire()
print 'Second try:', lock.acquire()

print "print this if not blocked..."

Output:

First try : True
Second try:

So, as in the example above, in a situation where separate code from the same thread needs to “re-acquire” the lock, we need to use threading.RLock instead of simple threading.Lock():

import threading

lock = threading.RLock()

print 'First try :', lock.acquire()
print 'Second try:', lock.acquire(0)

Output:

First try : True
Second try: 1

The output shows that we’re able to “re-acquire” the lock.

Note: also read about Lock Object in Python

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