Advanced Functions Part 2

Mutable Structures¶

There are some exceptions with scope to be aware of, and one example is lists. They are mutable meaning when we pass in a list to a function we are actually passing a reference to it in the memory of our machine. If we change it within a function, we change it everywhere!

If we have a function that doubles the first value in a list each time then prints it, we can see that the number is actually being changed outside the function.

In [16]:

#This is going to actually double the elements outside the function
def double(l):
    l[0] = l[0]*2
    print(l)

l = [1,2,3]
double(l)
double(l)
double(l)

[2, 2, 3]
[4, 2, 3]
[8, 2, 3]

Copying a List¶

If you want to pass in a copy of a list instead of the actual list, you can either pass it as l[:] or l.copy() and then anything that happens to the list in the function won't change the original list.

In [17]:

#The way to work around this is to give a copy of the list or a slice of the list, for example....
l = [1,2,3]
double(l[:])
double(l.copy())
double(l[:])
double(l.copy())

[2, 2, 3]
[2, 2, 3]
[2, 2, 3]
[2, 2, 3]

Nested Functions¶

Functions may be nested within one and if you would like. The following definition will define for us f1 which within its scope defines f2, a function to square something. Then we multiply the argument a by 2, and then apply our defined function before returning it.

In [18]:

#It is also possible to define functions within functions, so for this we have f1, and also f2 defined within it
def f1(a):
    def f2(b):
        return b**2
    a = a*2
    a = f2(a)
    return a

In [19]:

#So we multiply a by 2, then square it with f2
print(f1(3))

When you nest a function it is not defined outside the function scope. The code below fails for this reason.

In [20]:

#These functions are NOT defined outside of the f1 function however
#So you'll get an error
f2(3)


---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-20-e7ba8d554e48> in <module>
      1 #These functions are NOT defined outside of the f1 function however
      2 #So you'll get an error
----> 3 f2(3)

NameError: name 'f2' is not defined

Lambda Functions¶

Lambda functions are a way to define in short hand a function. They become very useful when we need simple one line functions. The format is:

y = lambda x: f(x)

where y then becomes the function you created. Below is a function which creates y, a function to square any input.

In [21]:

#Finally, lambda is a short hand way to define a function
#Call lambda x: followed by what you want x to be returned as
y = lambda x: x**2

In [22]:

#So we now square 2 with the y function
print(y(2))

Basics

Python Primer

Advanced Functions Part 2

Mutable Structures¶

Copying a List¶

Nested Functions¶

Lambda Functions¶

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