Lecture Notes 21: File IO

What are files?

Files are collections of values saved under a known address.

Internally, they are just 0s and 1s, but they can be interpreted as different things, like ASCII characters in a text file or just as 0s and 1s in a binary file.

When we want to interact with a file, we create a file object by providing the file's path.



A file object (more on objects later) has the file's information and allows the user to make use of a series of object methods to interact with that data.

Working with files

Opening and Closing

my_file = open('readme.txt') 

• if you want to write to it, we'll need to use the 'w' flag or the '+' flag
• if you don;t want to overwrite the file, and you just want to append (add), use the 'a' flag
• the flag 'x' is useful when you want a single copy of a file within a directory

Reading

# Read file contents
print ('Reading in data...')
f = open('example.txt')
lines = f.readlines()
f.close()

# Iterate over each line
for ln in lines:
    print("### {} ###".format(ln))

f = open('example.txt')

for line in f:
    print(line, end="")

f.close()

Writing

f = open("example.txt", "w")
print('Opening file example.txt for writing')
f.write('Joke #2: \nThe generation of random numbers is too important to be left to chance.')  # Write string
f.close()  # Close the file

f = open('example.txt')
contents = f.read()
print(contents)
f.close()

Appending!

f = open("example.txt", "a")
print('Opening file example.txt for appending')
f.write('Joke #1: \nThere are only 10 kinds of people in this world:\nthose who know binary and those who don’t.')  # Write string
f.close()  # Close the file

f = open('example.txt')
contents = f.read()
print(contents)
f.close()

The with operator

# Open a file for reading and appending
with open('example.txt', 'r+') as f:
  # Read in two integers
  lines = f.readlines()


print ("lines:\n",lines)
print()

print ("BEFORE:")
for idx in range(len(lines)):
  lines[idx] = lines[idx].replace("\n", "")
  print(lines[idx])
  
print()

# move line 2 to 0, 1, and 2
new_lines = []
new_lines.append(lines[2]+"\n")
new_lines.append(lines[3]+"\n")
new_lines.append(lines[4]+"\n")
new_lines.append(lines[0]+"\n")
new_lines.append(lines[1]+"\n")

print ("new lines:\n",new_lines)
print()


# and write everything back
with open('example.txt', 'w+') as f:
  f.writelines( new_lines )

# Open a file for reading and appending
with open('example.txt', 'r+') as f:
  # Read in two integers
  lines = f.readlines()

print ("AFTER:")
for li in lines:
  li = li.replace("\n", "")
  print(li)
  
print()

# No need to call f.close() - f closed automatically 
print('Closed example.txt')

Parsing: Practice with Files and Debugging

In the remaining time, let's work on combining File I/O and String Parsing.

First, open This Link

Now, open

Check out smith-map.

OSM map structure

• the osm file format is similr to that of html or xml: which follows a tree structure to describe things
  
  
  something like:
  
  top level: way
     |
     |
     | --- middle level: node
     |
     | --- middle level: node
            |
            | --- low level: tag
  
  would look like this:
  
  < way>
    < node\>
    < node>
      < tag\>
    <\node>
  <\way>
  
  
  
  
  (More details here)
  
  

  • • simple elements are surrounded by open tags: "<" and close tags: "\>"
    • complex objects are initiated by an openin element, like : "<node >"
      and closed by a closing element, like "<\node>"
  • every map has many points called nodes
  • a node can be simple or complex
    
    • a simple node has all its attributes in a single line
    • a complex node has tag elements
  • A way is a complex element composed of many nodes (a path is just a connection of points)

Additional readings

Check out this article on working with files

Homework