Usage of global variables with numba

numba_global.py

from numba import jit, njit

# Define a global variable.
g = 0

# foo doesn't care about the value of g at the time of definition.
@njit
def foo():
    # foo uses the global variable.
    return g

# foo considers the value of g at the time of compilation.
# Note that here foo is implicitly compiled because it is called for the first time.
g = 1
print(foo()) # -> 1

# foo doesn't see changes in g.
g = 2
print(foo()) # -> 1

# You need to explicitly recompile in order to update the value of g used by foo.
foo.recompile()
print(foo()) # -> 2

# MORAL OF THE STORY:
# never change the value of global variables and you won't face surprises!