The commands send and recv and their buffer counterparts Send and Recv are so-called blocking commands. That means a command send is completed when the corresponding send buffer is freed. When this will happen depends on several factors such as the particular communication architecture of your system and the amount of data that is to be communicated. Finally, the command send is considered to be freed when the corresponding command recv has got all the information. Without such a command recv, it will wait forever. This is called a deadlock situation.
The following script demonstrates a situation with the potential for deadlock. Both processes send simultaneously. If the amount of data to be communicated is too big to be stored the command send is waiting for a corresponding recv to empty the pipe, but recv never is invoked due to the waiting state. That's a deadlock.
from mpi4py...