# we define a variable in the global scope

In [28]: a = 5

# use that variable in a fucntion

In [29]: def add(x):

....: return x + a

# call the function -- a is used.

In [30]: add(3)

Out[30]: 8

# change a

In [31]: a = 12

# now what?

In [32]: add(3)

Out[32]: 15

# the new a is used

# but what if I don't want the results to depend on what a gets re-set to.

# but I don't want a constant, either.

# set a keyword argument:

In [34]: def add(x, a=a):

return x + a

....:

# try it:

In [35]: add(3)

Out[35]: 15

# it used the last a value

# reset a

In [36]: a = 100

# try it

In [37]: add(3)

Out[37]: 15

# still using that earlier value!

## Lesson: the keyword arguments are evaluted _when the function is defined_, NOT when it is called.