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Notes/07_Advanced_Topics/01_Variable_arguments.md

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+# 7.1 Variable Arguments
+
+### Positional variable arguments (*args)
+
+A function that accepts *any number* of arguments is said to use variable arguments.
+For example:
+
+```python
+def foo(x, *args):
+    ...
+```
+
+Function call.
+
+```python
+foo(1,2,3,4,5)
+```
+
+The arguments get passed as a tuple.
+
+```python
+def foo(x, *args):
+    # x -> 1
+    # args -> (2,3,4,5)
+```
+
+### Keyword variable arguments (**kwargs)
+
+A function can also accept any number of keyword arguments.
+For example:
+
+```python
+def foo(x, y, **kwargs):
+    ...
+```
+
+Function call.
+
+```python
+foo(2,3,flag=True,mode='fast',header='debug')
+```
+
+The extra keywords are passed in a dictionary.
+
+```python
+def foo(x, y, **kwargs):
+    # x -> 2
+    # y -> 3
+    # kwargs -> { 'flat': True, 'mode': 'fast', 'header': 'debug' }
+```
+
+### Combining both
+
+A function can also combine any number of variable keyword and non-keyword arguments.
+Function definition.
+
+```python
+def foo(*args, **kwargs):
+    ...
+```
+
+This function takes any combination of positional or keyword arguments.
+It is sometimes used when writing wrappers or when you want to pass arguments through to another function.
+
+### Passing Tuples and Dicts
+
+Tuples can be expanded into variable arguments.
+
+```python
+numbers = (2,3,4)
+foo(1, *numbers)      # Same as f(1,2,3,4)
+```
+
+Dictionaries can also be expaded into keyword arguments.
+
+```python
+options = {
+    'color' : 'red',
+    'delimiter' : ',',
+    'width' : 400
+}
+foo(data, **options)
+# Same as foo(data, color='red', delimiter=',', width=400)
+```
+
+These are not commonly used except when writing library functions.
+
+## Exercises
+
+### (a) A simple example of variable arguments
+
+Try defining the following function:
+
+```python
+>>> def avg(x,*more):
+        return float(x+sum(more))/(1+len(more))
+
+>>> avg(10,11)
+10.5
+>>> avg(3,4,5)
+4.0
+>>> avg(1,2,3,4,5,6)
+3.5
+>>>
+```
+
+Notice how the parameter `*more` collects all of the extra arguments.
+
+### (b) Passing tuple and dicts as arguments
+
+Suppose you read some data from a file and obtained a tuple such as
+this:
+
+```
+>>> data = ('GOOG', 100, 490.1)
+>>>
+```
+
+Now, suppose you wanted to create a `Stock` object from this
+data.  If you try to pass `data` directly, it doesn't work:
+
+```
+>>> from stock import Stock
+>>> s = Stock(data)
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+TypeError: __init__() takes exactly 4 arguments (2 given)
+>>> 
+```
+
+This is easily fixed using `*data` instead.  Try this:
+
+``python
+>>> s = Stock(*data)
+>>> s
+Stock('GOOG', 100, 490.1)
+>>>
+```
+
+If you have a dictionary, you can use `**` instead. For example:
+
+```python
+>>> data = { 'name': 'GOOG', 'shares': 100, 'price': 490.1 }
+>>> s = Stock(**data)
+Stock('GOOG', 100, 490.1)
+>>>
+```
+
+### (c) Creating a list of instances
+
+In your `report.py` program, you created a list of instances
+using code like this:
+
+```python
+def read_portfolio(filename):
+    '''
+    Read a stock portfolio file into a list of dictionaries with keys
+    name, shares, and price.
+    '''
+    with open(filename) as lines:
+        portdicts = fileparse.parse_csv(lines, 
+                               select=['name','shares','price'], 
+                               types=[str,int,float])
+
+    portfolio = [ Stock(d['name'], d['shares'], d['price']) 
+                  for d in portdicts ]
+    return Portfolio(portfolio)
+```
+
+You can simplify that code using `Stock(**d)` instead.  Make that change.
+
+### (d) Argument pass-through
+
+The `fileparse.parse_csv()` function has some options for changing the
+file delimiter and for error reporting.  Maybe you'd like to expose those
+options to the `read_portfolio()` function above.   Make this change:
+
+```
+def read_portfolio(filename, **opts):
+    '''
+    Read a stock portfolio file into a list of dictionaries with keys
+    name, shares, and price.
+    '''
+    with open(filename) as lines:
+        portdicts = fileparse.parse_csv(lines, 
+                                        select=['name','shares','price'], 
+                                        types=[str,int,float],
+                                        **opts)
+
+    portfolio = [ Stock(**d) for d in portdicts ]
+    return Portfolio(portfolio)
+```
+
+Once you've made the change, trying reading a file with some errors:
+
+```python
+>>> import report
+>>> port = report.read_portfolio('Data/missing.csv')
+Row 4: Couldn't convert ['MSFT', '', '51.23']
+Row 4: Reason invalid literal for int() with base 10: ''
+Row 7: Couldn't convert ['IBM', '', '70.44']
+Row 7: Reason invalid literal for int() with base 10: ''
+>>>
+```
+
+Now, try silencing the errors:
+
+```python
+>>> import report
+>>> port = report.read_portfolio('Data/missing.csv', silence_errors=True)
+>>>
+```
+
+[Next](02_Anonymous_function)