Section 1

Notes/01_Introduction/06_Files.md

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+# 1.6 File Management
+
+## Notes
+
+### File Input and Output
+
+Open a file.
+
+```python
+f = open('foo.txt', 'rt')     # Open for reading (text)
+g = open('bar.txt', 'wt')     # Open for writing (text)
+```
+
+Reading data.
+
+```python
+data = f.read()
+
+# Read only up to 'maxbytes' bytes
+data = f.read([maxbytes])
+```
+
+Writing text to a file.
+
+```python
+g.write('some text')
+```
+
+Close when you are done.
+
+```python
+f.close()
+g.close()
+```
+
+Files should be properly closed. This is why the preferred approach is to use the `with` statement.
+
+```python
+with open(filename, 'rt') as f:
+    # Use the file `f`
+    ...
+    # No need to close explicitly
+...statements
+```
+
+This automatically closes the file when control leaves the indented code block.
+
+### Common Idioms for Reading File Data
+
+Reading an entire file all at once as a string.
+
+```python
+with open('foo.txt', 'rt') as f:
+    data = f.read()
+    # `data` is a string with all the text in `foo.txt`
+```
+
+Reading a file line-by-line
+
+```python
+with open(filename, 'rt') as f:
+    for line in f:
+        # Process the line `f`
+```
+
+Writing string data.
+
+```python
+with open('outfile', 'wt') as f:
+    f.write('Hello World\n')
+    ...
+```
+
+Redirecting the print function.
+
+```python
+with open('outfile', 'wt') as f:
+    print('Hello World', file=f)
+    ...
+```
+
+## Exercises 1.6
+
+This exercise depends on a file `Data/portfolio.csv`.  The file contains a list of lines with information on a portfolio of stocks.
+Locate the file and look at its contents:
+
+### (a) File Preliminaries
+
+*Note: Make sure you are running Python in a location where you can access the `portfolio.csv` file.
+You can find out where Python thinks it's running by doing this:
+
+```python
+>>> import os
+>>> os.getcwd()
+'/Users/beazley/Desktop/practical-python' # Output vary
+>>>
+```
+
+First, try reading the entire file all at once as a big string:
+
+```pycon
+>>> with open('Data/portfolio.csv', 'rt') as f:
+        data = f.read()
+>>> data
+'name,shares,price\n"AA",100,32.20\n"IBM",50,91.10\n"CAT",150,83.44\n"MSFT",200,51.23\n"GE",95,40.37\n"MSFT",50,65.10\n"IBM",100,70.44\n'
+>>> print(data)
+name,shares,price
+"AA",100,32.20
+"IBM",50,91.10
+"CAT",150,83.44
+"MSFT",200,51.23
+"GE",95,40.37
+"MSFT",50,65.10
+"IBM",100,70.44
+>>>
+```
+
+In the above example, it should be noted that Python has two modes of output.
+In the first mode where you type `data` at the prompt, Python shows you the raw string representation including quotes and escape codes.
+When you type `print(data)`, you get the actual formatted output of the string.
+
+Although reading a file all at once is simple, it is often not the
+most appropriate way to do it—especially if the file happens to be
+huge or if contains lines of text that you want to handle one at a
+time.
+
+To read a file line-by-line, use a for-loop like this:
+
+```pycon
+>>> with open('Data/portfolio.csv', 'rt') as f:
+    for line in f:
+        print(line, end='')
+name,shares,price
+"AA",100,32.20
+"IBM",50,91.10
+...
+>>>
+```
+
+When you use this code as shown, lines are read until the end of the file is reached at which point the loop stops.
+
+On certain occasions, you might want to manually read or skip a *single* line of text (e.g., perhaps you want to skip the first line of column headers).
+
+```pycon
+>>> f = open('Data/portfolio.csv', 'rt')
+>>> headers = next(f)
+>>> headers
+'name,shares,price\n'
+>>> for line in f:
+    print(line, end='')
+
+"AA",100,32.20
+"IBM",50,91.10
+...
+>>> f.close()
+>>>
+```
+
+`next()` returns the next line of text in the file. If you were to call it repeatedly, you would get successive lines. 
+However, just so you know, the `for` loop already uses `next()` to obtain its data.
+Thus, you normally wouldn’t call it directly unless you’re trying to explicitly skip or read a single line as shown.
+
+Once you’re reading lines of a file, you can start to perform more processing such as splitting.
+For example, try this:
+
+```pycon
+>>> f = open('Data/portfolio.csv', 'rt')
+>>> headers = next(f).split(',')
+>>> headers
+['name', 'shares', 'price\n']
+>>> for line in f:
+    row = line.split(',')
+    print(row)
+
+['"AA"', '100', '32.20\n']
+['"IBM"', '50', '91.10\n']
+...
+>>> f.close()
+```
+
+*Note: In these examples, `f.close()` is being called explicitly because the `with` statement isn’t being used.*
+
+### (b) Reading a data file
+
+Now that you know how to read a file, let’s write a program to perform a simple calculation.
+
+The columns in `portfolio.csv` correspond to the stock name, number of
+shares, and purchase price of a single share.  Write a program called
+`pcost.py` that opens this file, reads all lines, and calculates how
+much it cost to purchase all of the shares in the portfolio.
+
+*Hint: to convert a string to an integer, use `int(s)`. To convert a string to a floating point, use `float(s)`.*
+
+Your program should print output such as the following:
+
+```bash
+Total cost 44671.15
+```
+
+### (c) Other kinds of 'files'
+
+What if you wanted to read a non-text file such as a gzip-compressed datafile?
+The builtin `open()` function won’t help you here, but Python has a library module `gzip` that can read gzip compressed files.
+
+Try it:
+
+```pycon
+>>> import gzip
+>>> with gzip.open('Data/portfolio.csv.gz') as f:
+    for line in f:
+        print(line, end='')
+
+... look at the output ...
+>>>
+```
+
+[Next]("07_Functions.html")