ezimms99 / 608-mod6

608-mod6

Chapter 9 Notes:

Introduction:

• Files provide long term retention of typically large amounts of data.
• Data maintained in files is persistent.
• Store files on secondary storage devices.
• Exception Handling
  • An exception indicates an execution-time problem.

Files:

• Text file - Sequence of characters
• Binary file - sequence of bytes
• end-of-file-marker
  • Denotes the end of a file
• Standard file objects:
  • sys.stdin - standard input file object
  • sys.stdout - standard output file object
  • sys.stderr - standard error file object

Text-File Processing:

• Python's with statement
  • Acquires a resource and assigns its corresponding object to a variable
  • Allows the application to use the resource via that variable
  • Calls the resource objects close method to release the resource when program control reaches end.
  • Built-in open function opens the file contents.
  • Mode argument specifies the file-open mode.
  • .txt indicates plain text file.
  • As clause assigns the variable the name
• Reading data from a text file:
  • SEE JUPYTER LAB FILE
  • Argument 'r' is used to read the file.
  • File objects readlines method also can be used to read an entire text file.
  • Use function seek in order to locate a specific file position.

Updating Text Files:

• Steps to update: See Jupyter Lab examples
• os module provides functions for interacting with operating system.

Sterilization with JSON:

• JSON objects contain a comma-separated list.
• JSON supports arrays
• json module enables you to convert objects to JSON --> This is known as steralizing data.
• The dump function steralizes a file to json.
• Can also desteralize json
  • load function reads the entire JSON contents of its file object argument and converts the JSON into a python object.
  • Can also use [] to locate a specific index in a JSON file.

Focus on Security: pickle Serialization and Deserialization

• Python's pickle module can serialize objects in a python specific data format.
• Pickle files can be hacked.
• legacy code - old code that's often no longer supported.

Additional Notes Regarding Files:

• Page 331 contains a chart for the different file modes.
• read method returns a string containing the number of characters specified by the method's integer argument.
• readline method returns one line of text as a string.
• writelines method recieves a list of strings and writes its contents to a file.
• The classes that python uses to create file objects are defined in the python standard library's io module.

Handling Exceptions:

• A FileNotFoundError occurs if you attempt to open a non-existent file for reading.
• A PermissionsError occurs if you attempt an operation for which you do not have permission.
• Division by zero
  • Invalid input resulting in a ZeroDivisionError
  • This ZeroDivisionError exception is raised an displayed.
  • A ValueError occurs if you attempt to convert to to an integer a string.
• Try statements
  • enables exception handeling
  • A try clause may be followed by an except clause to handle exceptions.
  • the else clause only executes if no exceptions occur.
• Point in program where exception occurs is the raise point.
• Catching Multiple Exceptions in One except clause
  • Put all your except statements in a tuple.

Finally Clause:

• Closing a file helps prevent a resource leak in which the file resource is not available to other programs.
• finally clause is guaranteed to execute, regardless of whether its try suite executes successfully.

Explicitly Raising an Exception:

• The raise statement explicitly raises an exception.

(Optional) Stack Unwinding and Tracebacks:

• Includes the lines of code that lead to the exception.
• Traceback shows the type of exception that occured followed by the complete function call stack.
• When an exception is not caught in a given function, stack unwinding occurs.

Intro to Data Science: Working with CSV Files:

• CSV (Comma-separated values)
• The csv module provides functions for working with CSV files.
• writerows can replace all writerow values.
• csv's reader function returns an object that reads a CSV-format dat from the specified file object.
• Be careful with commas in a CSV file.
• Reading CSV Files into Pandas DataFrames
  • The popular Rdatasets repository provides links over 1100 free datasets in comma-separated values format.
• Data method 'head' returns the first five rows.
• data method 'tail' returns final five rows.

Chapter 17 Notes: Big data - Hadoop, Spark, NoSQL, and IoT

SQL:

• Structured Query Language

A books Database:

• read_sql executes a sql query and returns a data frame containing results.
• SELECT gets rows and columns from tables.
• • grabs everything from the table.
• FROM is the place you are grabbing the data.

SELECT Queries:

• Can insert title names of columns to grab specific columns from the data.

WHERE clause:

• Specifies the results of a query.
• Like is used for pattern matching, searching for strings that match a given pattern.
• Using % specifies a wildcard position to autofill any possibilities.
• Using an _ indicates a single wildcard character.

ORDER BY clause:

• Sorts the queries results in a specified order.
• Can also order by multiple columns.
• Can also change which columns are ordered what way.

Merging Data From Multiple Tables:

• Inner Join: Joins matching data from two tables.

INSERT INTO:

• Add data to a SQL table.

UPDATE:

• Modifies existing data

DELETE FROM:

• Deletes data from a table.