Dictionaries And Files

Dictionaries

Dictionaries are a way to store data as unordered key-value pairs. This is extremely useful to represent attributes of a single thing.

Features:

• Keys are usually strings or ints

• Values can be any type

• Mutable

Initialization:

person = { "name": "Dan", "age": 29 }

Operations:

person = { "name": "Dan", "age": 29 }

# Element access
person["name"]
person["social_insurance_num"] # throws KeyError
person.get("social_insurance_num") # returns None
person.get("social_insurance_num", 0) # returns fallsback 0

# Element count
len(person) # returns 2

# Convert to list
keys = person.keys() # returns list of keys (as a view object)
key_values = person.items() # returns tuples of key-value pairs (as a view object)

# Iteration
for key in person:
  # ...

# Element mutation
person["name"] = "John" # edit
del person["name"] # remove

# Element check
"name" in person # returns False

# Add attributes from another dictionary
person.update({ "location": "Canada" })

Files

Opening

To open a file, use the following command:

stream = open(path_to_file, mode)

where mode can be one of

• "r" to read file (default mode)

  • Returns error if file doesn't exist

• "w" to write to file

  • Removes all old data if file exists

  • Creates file if it doesn't exist

• "a" to append to end of file

  • Creates file if it doesn't exist

• "r+" to read and write at the same time

  • Returns error if file doesn't exist

  • Does NOT remove old data if file exists

Reading

The following functions are available for reading files when you have a stream available:

• stream.read() reads all text

• stream.readline() reads one line

• stream.readlines() reads all lines as a list

Pro tip: When reading lines, you'll often have whitespace characters like at the end. Use the string method rstrip to remove them.

Writing

The following functions are available for writing lines to a file when you have a stream available:

• stream.write(string) writes text

• stream.writelines(list_of_strings) writes multiple lines

Closing

Any time you finish reading/writing/appending to a file, you must remember to close the file. Otherwise, you risk having incomplete changes.

There are 2 ways to close a file:

# Imperatively
stream.close()

# Using `with` statement
with open(path_to_file, "r") as stream:
  # closes automatically after executing code block