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David Beazley
2020-05-26 11:40:09 -05:00
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Notes/01_Introduction/07_Functions.md
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# 1.7 Introduction to Functions
# 1.7 Functions
As your programs start to get larger, you'll want to get organized. This section
introduces functions. Error handling with exceptions is also introduced.
@@ -41,14 +41,15 @@ x = math.sqrt(10)
# `urllib.request` module
import urllib.request
u = urllib.request.urlopen('http://www.python.org/') data = u.read()
u = urllib.request.urlopen('http://www.python.org/')
data = u.read()
```
We will cover libraries and modules in more detail later.
### Errors and exceptions
Errors are reported as exceptions. An exception causes the program to stop.
Functions report errors as exceptions. An exception causes the program to stop.
Try this in your python REPL.
@@ -60,13 +61,14 @@ ValueError: invalid literal for int() with base 10: 'N/A'
>>>
```
For debugging purposes, the message describes what happened, where the error occurred and the traceback.
For debugging purposes, the message describes what happened, where the error occurred,
and a traceback showing the other function calls that led to the failure.
### Catching and Handling Exceptions
Exceptions can be caught and handled.
To catch, use `try - except` statement.
To catch, use the `try - except` statement.
```python
for line in f:
@@ -88,7 +90,7 @@ To raise an exception, use the `raise` statement.
raise RuntimeError('What a kerfuffle')
```
This will cause the program to abord with an exception traceback. Unless caught by a `try-except` block.
This will cause the program to abort with an exception traceback. Unless caught by a `try-except` block.
```bash
% python3 foo.py
@@ -100,7 +102,7 @@ RuntimeError: What a kerfuffle
## Exercises
### (a) Defining a function
### Exercise 1.29: Defining a function
You can define a simple function using the `def` statement. For example,
@@ -119,12 +121,15 @@ Hello Paula
If the first statement of a function is a string, it serves as documentation.
Try typing a command such as `help(greeting)` to see it displayed.
### (b) Turning a script into a function
### Exercise 1.30: Turning a script into a function
Take the code you wrote for the `pcost.py` program in the last exercise and turn it into a function `portfolio_cost(filename)`.
The function takes a filename as input, reads the portfolio data in that file, and returns the total cost of the portfolio.
Take the code you wrote for the `pcost.py` program in [Exercise 1.27](06_Files)
and turn it into a function `portfolio_cost(filename)`. The
function takes a filename as input, reads the portfolio data in that
file, and returns the total cost of the portfolio as a float.
To use your function, change your program so that it looks something like this:
To use your function, change your program so that it looks something
like this:
```python
def portfolio_cost(filename):
@@ -137,7 +142,8 @@ print('Total cost:', cost)
```
When you run your program, you should see the same output as before.
After you’ve run your program, you can also call your function interactively by typing this:
After you’ve run your program, you can also call your function
interactively by typing this:
```bash
bash $ python3 -i pcost.py
@@ -151,7 +157,8 @@ This will allow you to call your function from the interactive mode.
>>>
```
Being able to experiment with your code interactively is useful for testing and debugging.
Being able to experiment with your code interactively is useful for
testing and debugging.
What happens if you try your function on a file with some missing fields?
@@ -170,14 +177,15 @@ you can either sanitize the original input file by eliminating bad
lines or you can modify your code to handle the bad lines in some
manner.
Modify the `pcost.py` program to catch the exception, print a warning message,
and continue processing the rest of the file.
Modify the `pcost.py` program to catch the exception, print a warning
message, and continue processing the rest of the file.
### (c) Using a library function
### Exercise 1.31: Using a library function
Python comes with a large standard library of useful functions.
One library that might be useful here is the `csv` module. You should use it whenever you have to work with CSV data files.
Here is an example of how it works:
Python comes with a large standard library of useful functions. One
library that might be useful here is the `csv` module. You should use
it whenever you have to work with CSV data files. Here is an example
of how it works:
```pycon
>>> import csv
@@ -200,12 +208,15 @@ Here is an example of how it works:
>>>
```
One nice thing about the `csv` module is that it deals with a variety of low-level details such as quoting and proper comma splitting.
In the above output, you’ll notice that it has stripped the double-quotes away from the names in the first column.
One nice thing about the `csv` module is that it deals with a variety
of low-level details such as quoting and proper comma splitting. In
the above output, you’ll notice that it has stripped the double-quotes
away from the names in the first column.
Modify your `pcost.py` program so that it uses the `csv` module for parsing and try running earlier examples.
Modify your `pcost.py` program so that it uses the `csv` module for
parsing and try running earlier examples.
### (d) Reading from the command line
### Exercise 1.32: Reading from the command line
In the `pcost.py` program, the name of the input file has been hardwired into the code:
@@ -221,9 +232,11 @@ cost = portfolio_cost('Data/portfolio.csv')
print('Total cost:', cost)
```
That’s fine for learning and testing, but in a real program you probably wouldn’t do that.
That’s fine for learning and testing, but in a real program you
probably wouldn’t do that.
Instead, you might pass the name of the file in as an argument to a script. Try changing the bottom part of the program as follows:
Instead, you might pass the name of the file in as an argument to a
script. Try changing the bottom part of the program as follows:
```python
# pcost.py