@@ -171,6 +171,135 @@ print(new_abs_function(3))

171171print(new_abs_function(-3))172172```173173174+### Keyword Arguments175+176+```{index} single: Python; keyword arguments177+```178+179+In a {ref}`previous lecture <python_by_example>`, you came across the statement180+181+```{code-block} python3182+:class: no-execute183+184+plt.plot(x, 'b-', label="white noise")185+```186+187+In this call to Matplotlib's `plot` function, notice that the last argument is passed in `name=argument` syntax.188+189+This is called a *keyword argument*, with `label` being the keyword.190+191+Non-keyword arguments are called *positional arguments*, since their meaning192+is determined by order193+194+* `plot(x, 'b-', label="white noise")` is different from `plot('b-', x, label="white noise")`195+196

+Keyword arguments are particularly useful when a function has a lot of arguments, in which case it's hard to remember the right order.

197+198+You can adopt keyword arguments in user-defined functions with no difficulty.199+200+The next example illustrates the syntax201+202+```{code-cell} python3203+def f(x, a=1, b=1):204+ return a + b * x205+```206+207+The keyword argument values we supplied in the definition of `f` become the default values208+209+```{code-cell} python3210+f(2)211+```212+213+They can be modified as follows214+215+```{code-cell} python3216+f(2, a=4, b=5)217+```218+219+### The Flexibility of Python Functions220+221+As we discussed in the {ref}`previous lecture <python_by_example>`, Python functions are very flexible.222+223+In particular224+225+* Any number of functions can be defined in a given file.226+* Functions can be (and often are) defined inside other functions.227+* Any object can be passed to a function as an argument, including other functions.228+* A function can return any kind of object, including functions.229+230+We already {ref}`gave an example <test_program_6>` of how straightforward it is to pass a function to231+a function.232+233+Note that a function can have arbitrarily many `return` statements (including zero).234+235+Execution of the function terminates when the first return is hit, allowing236+code like the following example237+238+```{code-cell} python3239+def f(x):240+ if x < 0:241+ return 'negative'242+ return 'nonnegative'243+```244+245+Functions without a return statement automatically return the special Python object `None`.246+247+248+### Docstrings249+250+```{index} single: Python; Docstrings251+```252+253+Python has a system for adding comments to functions, modules, etc. called *docstrings*.254+255+The nice thing about docstrings is that they are available at run-time.256+257+Try running this258+259+```{code-cell} python3260+def f(x):261+ """262+ This function squares its argument263+ """264+ return x**2265+```266+267+After running this code, the docstring is available268+269+```{code-cell} ipython270+f?271+```272+273+```{code-block} ipython274+:class: no-execute275+276+Type: function277+String Form:<function f at 0x2223320>278+File: /home/john/temp/temp.py279+Definition: f(x)280+Docstring: This function squares its argument281+```282+283+```{code-cell} ipython284+f??285+```286+287+```{code-block} ipython288+:class: no-execute289+290+Type: function291+String Form:<function f at 0x2223320>292+File: /home/john/temp/temp.py293+Definition: f(x)294+Source:295+def f(x):296+ """297+ This function squares its argument298+ """299+ return x**2300+```301+302+With one question mark we bring up the docstring, and with two we get the source code as well.174303175304### One-Line Functions: `lambda`176305

@@ -351,6 +480,7 @@ In the context of our program, the ability to bind new names to functions

351480means that there is no problem *passing a function as an argument to another352481function*---as we did above.353482483+354484## Exercises355485356486```{exercise}