Section 1
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David Beazley
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Notes/01_Introduction/07_Functions.md
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Notes/01_Introduction/07_Functions.md
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# 1.7 Introduction to Functions
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## Notes
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### Custom Functions
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Use functions for code you want to reuse. Here is a function definition:
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```python
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def sumcount(n):
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'''
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Returns the sum of the first n integers
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'''
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total = 0
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while n > 0:
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total += n
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n -= 1
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return total
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```
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Calling a function.
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```python
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# Use parenthesis to call the function
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a = sumcount(100)
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```
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A function is a series of statements that perform some task and return a result.
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The `return` keyword is needed to explicitly specify the return value of the function.
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### Library Functions
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Python comes with a large standard library.
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Library modules are accessed using `import`.
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```python
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# `math` module
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import math
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x = math.sqrt(10)
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# `urllib.request` module
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import urllib.request
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u = urllib.request.urlopen('http://www.python.org/') data = u.read()
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```
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We will cover libraries and modules in more detail later.
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### Errors and exceptions
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Errors are reported as exceptions. An exception causes the program to stop.
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Try this in your python REPL.
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```pycon
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>>> int('N/A')
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Traceback (most recent call last):
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File "<stdin>", line 1, in <module>
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ValueError: invalid literal for int() with base 10: 'N/A'
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>>>
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```
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For debugging purposes, the message describes what happened, where the error occurred and the traceback.
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### Catching and Handling Exceptions
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Exceptions can be caught and handled.
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To catch, use `try - except` statement.
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```python
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for line in f:
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fields = line.split()
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try:
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shares = int(fields[1])
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except ValueError:
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print("Couldn't parse", line)
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...
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```
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The name `ValueError` must match the kind of error you are trying to catch.
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### Raising Exceptions
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To raise an exception, use the `raise` statement.
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```python
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raise RuntimeError('What a kerfuffle')
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```
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This will cause the program to abord with an exception traceback. Unless caught by a `try-except` block.
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```bash
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% python3 foo.py
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Traceback (most recent call last):
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File "foo.py", line 21, in <module>
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raise RuntimeError("What a kerfuffle")
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RuntimeError: What a kerfuffle
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```
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## Exercises 1.7
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### (a) Defining a function
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You can define a simple function using the `def` statement. For example,
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```python
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>>> def greeting(name):
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'Issues a greeting'
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print('Hello', name)
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>>> greeting('Guido')
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Hello Guido
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>>> greeting('Paula')
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Hello Paula
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>>>
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```
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If the first statement of a function is a string, it serves as documentation.
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Try typing a command such as `help(greeting)` to see it displayed.
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### (b) Turning a script into a function
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Take the code you wrote for the `pcost.py` program in the last exercise and turn it into a function `portfolio_cost(filename)`.
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The function takes a filename as input, reads the portfolio data in that file, and returns the total cost of the portfolio.
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To use your function, change your program so that it looks something like this:
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```python
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def portfolio_cost(filename):
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...
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# Your code here
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...
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cost = portfolio_cost('Data/portfolio.csv')
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print('Total cost:', cost)
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```
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When you run your program, you should see the same output as before.
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After you’ve run your program, you can also call your function interactively by typing this:
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```bash
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bash $ python3 -i pcost.py
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```
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This will allow you to call your function from the interactive mode.
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```python
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>>> portfolio_cost('Data/portfolio.csv')
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44671.15
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>>>
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```
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Being able to experiment with your code interactively is useful for testing and debugging.
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What happens if you try your function on a file with some missing fields?
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```pycon
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>>> portfolio_cost('Data/missing.csv')
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Traceback (most recent call last):
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File "<stdin>", line 1, in <module>
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File "pcost.py", line 11, in portfolio_cost
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nshares = int(fields[1])
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ValueError: invalid literal for int() with base 10: ''
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>>>
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```
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At this point, you’re faced with a decision. To make the program work
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you can either sanitize the original input file by eliminating bad
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lines or you can modify your code to handle the bad lines in some
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manner.
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Modify the `pcost.py` program to catch the exception, print a warning message,
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and continue processing the rest of the file.
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### (c) Using a library function
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Python comes with a large standard library of useful functions.
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One library that might be useful here is the `csv` module. You should use it whenever you have to work with CSV data files.
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Here is an example of how it works:
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```pycon
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>>> import csv
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>>> f = open('Data/portfolio.csv')
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>>> rows = csv.reader(f)
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>>> headers = next(rows)
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>>> headers
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['name', 'shares', 'price']
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>>> for row in rows:
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print(row)
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['AA', '100', '32.20']
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['IBM', '50', '91.10']
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['CAT', '150', '83.44']
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['MSFT', '200', '51.23']
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['GE', '95', '40.37']
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['MSFT', '50', '65.10']
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['IBM', '100', '70.44']
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>>> f.close()
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>>>
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```
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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.
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In the above output, you’ll notice that it has stripped the double-quotes away from the names in the first column.
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Modify your `pcost.py` program so that it uses the `csv` module for parsing and try running earlier examples.
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### (d) Reading from the command line
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In the `pcost.py` program, the name of the input file has been hardwired into the code:
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```python
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# pcost.py
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def portfolio_cost(filename):
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...
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# Your code here
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...
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cost = portfolio_cost('Data/portfolio.csv')
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print('Total cost:', cost)
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```
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That’s fine for learning and testing, but in a real program you probably wouldn’t do that.
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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:
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```python
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# pcost.py
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import sys
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def portfolio_cost(filename):
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...
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# Your code here
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...
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if len(sys.argv) == 2:
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filename = sys.argv[1]
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else:
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filename = 'portfolio.csv'
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cost = portfolio_cost(filename)
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print('Total cost:', cost)
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```
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`sys.argv` is a list that contains passed arguments on the command line (if any).
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To run your program, you’ll need to run Python from the
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terminal.
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For example, from bash on Unix:
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```bash
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bash % python3 pcost.py Data/portfolio.csv
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Total cost: 44671.15
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bash %
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```
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