Section 12: File IO

Question 1

What is maintaining data?

Answer 1

• Until now, program short and produces some output then terminates: data produced disappears
• Data in our program uses is either hard-coded in the program, or received as input from user

Maintaining data: reading and writing text files

> Text File: sequence of characters stored in a permanent medium

Steps when working with files:

1. Open file using build-in function open()
2. Read data from file
3. Close file

Question 2

Steps

How to open file using build-in function:

(1) fobj, (2) filename, (3) mode

Answer 2

```python
fobj = open(filename, mode)
~~~

• fobj: returns a file obejct use to read from file
• filename: string corresponds to the name of the file that want to open
  
  • if the file is in the current dictionary: use its name
  • if located somewhere elese: use full path to the file
• mode: Different permission modes:
  
  • ‘r’ for reading
  • ‘w’ for writing
  • ‘a’ for appending

Question 3

Steps: Read/Writing data from file

(1) Read a file

Answer 3

• read(size) method that works on file objectsTakes one optional argument (size: num of characters to read from the file → omitted, read whole thing)
• Returns string containing characters of the file

Ex:
```python
filenam = ‘quotes.txt’
fobj = open(filename, ‘r’) #more r for reading
file_content = fobj.read() #read the whole thing
print(file_content) #print the string
~~~

Question 4

Steps: Read/Writing data from file

(2) Reading a file line by line

Answer 4

• Object returned by open() is iterable sequence that produces the sequence of lines contained in the file
• Can use a for loop to go over each line in the file

ex:

```python
filenam = ‘quotes.txt’
fobj = open(filename, ‘r’) #more r for reading
for line in fobj: #file object is iterable
print(line)
fobj.close() #closing the file
~~~

Question 5

Steps: Read/Writing data from file

(3) Writing to a file

Answer 5

(1) Open file with open using the mode 'w' for ‘write’

• If the file does not exist, it will be created
• If the file does exist, it will be deleted and replaced with an empty file

(2) Can write(text) method on the file object to write the string text into the file

(3) Close the file

```python
filenam = ‘quotes.txt’
fobj = open(filename, ‘r’) #more r for reading
fobj.write(“More first line in a file!)
fobj. write(“This will appear on the second line(?)”)
fobj.close()
~~~

Question 6

Steps: Read/Writing data from file

Escape Characters

Answer 6

• In Python strings, the backslash \ is special character: escape character
  
  1. \t: tab character
  2. \n: newline character

```python
fobj.write(‘cherry\tkiwi’)
~~~

Question 7

Steps: Read/Writing data from file

Appending

Answer 7

(Add to the end of the file)

1. Open the file with open() using mode 'a' for ‘append’. If it does not exist, it is created
2. Call write(s) method on the file object to insert the string s at the end of the file
3. Close file

        python
    filename = "quotes.txt" # assume it exists from last slide
    fobj = open(filename, "a") # mode "a" for appending
    fobj.write("\nA third line!! Will the madness ever stop?")
    fobj.close()

Question 8

Steps

3. Close file

Answer 8

close() is a method that works on file objects. Takes no arguments and returns nothing. Closes the file

Why … ?
The OS may lock file till its close, too many open files may cause comp slow down, etc…

```python
filename = “quotes.txt”
fobj = open(filename, “r”) # mode “r” for reading
file_content = fobj.read() # read whole file
print(file_content) # print the string
fobj.close()
~~~

Question 9

Opening Modes

Answer 9

Can also add a + after the mode to indicate both reading and writing.
r+

Question 10

What is a Pickle Module

Answer 10

Another way to load/save data

> Pickle module lets us load/save any Python object from/to a file

• Resulting file only be opened by Python (not regular text editing program)

Question 11

What are the different parts of the Pickle Module:

Answer 11

1. import pickle module to use it
2. Can put any data (x) in pickle file
3. Name file using data.pkl extension
4. Use 'wb' mode (write in binary) → data written in form of byte objects
5. Use dump method to save data x in file
6. close() file

Question 12

Using pickle vs. regular file IO

Answer 12

• If writing data to a file and want other programs/people able to open file and read it → write directly in file using open/write/close
• If you are writing data to a file that only your program will use → use pickle

Question 13

What is CSV Formatting?

Answer 13

> Comma-Separate Values (CSV): file stores tabular data by sing commas to separate values

Ex: Movie 1,80,100,90,100 Movie 2,90,93,100,100 Movie 3,80,100,90,100

• Often used by program can export data
• Ex: Spreadsheet software (Excel, Numbers) can directly open CSV file

```python
»> read_csv(“movie_data.csv”)
[[“Movie 1”, “80”, “100”, “90”, “100”], [“Movie 2”, “90”, “93”,
“100”, “100”], [“Movie 3”, “80”, “100”, “90”, “100”]]
~~~