Files
CSCI 1470

by K. Yue

1. File Basics

• Files are used to store data permanently, independent of applications.
• Thus, many applications can use the same file to share data.
• Data in files are retained even when power is turned off.
• Files are stored in secondary storage devices, such as hard disks, flash drive, magnetic tapes, etc.
• Data is eventually stored in bits (0 or 1).
• Text files store a sequence of characters encoded in ASCII, UTF, or other encoding formats.
  • Eventually the encoding is stored in bytes and bits. Thus, text files are sort of binary too.
  • A Byte is just 8 bits, but in character encoding, each bit does not have an individual meaning.
• Binary files store data in non-textual and binary manner.

2. Streams and I/O Streams

• In computer science, the term streams refer to abstract objects (that need to be eventually implemented) for sequentially reading data from and writing data to various channels.
• These channels are usually used for Input and Output purposes (i.e., I/O streams)
• Like many other languages, Python have three standards I/O streams:
  1. standard input: sys.stdin
  2. standard output: sys.stdout
  3. standard error: sys.stderr
• For example, if you are running a Python program in a command terminal, all three standard I/O streams are the command terminal.
  • The command terminal may be considered as an implementation of the three standard I/O streams.

Example:

stream_1.py:

#   standard I/O stream in Python.

name = input("Your name: ")
print(f"hi, {name}.")

num1 = int(input("number 1: "))
num2 = int(input("number 2: "))
num3 = int(input("number 3: "))

print(f"num1/num2: {num1/num2}")
print(f"num1/num3: {num1/num3}")
 

• Streams can be implemented in many ways. Examples:
  1. Files
  2. Network connections.
  3. In memory buffers (such as strings, since strings are sequential).

3. Input files

3.1 Basics

• Basic commands for read a file is:
  • open(filename): open a file and returns a file object. 
  • file_1.read(): read and return the entire content of the file_1 object.
  • file_1.readline(): read and return the content of the file_1 object up to and include a end of line symbol ("\n"). If the end of file is reached, return "".
  • file_1.close(): close the file_1 object.

Example:

infile_1.py.txt:

# Open a file with mode "r" for read and returns a file object.
f = open("infile_1.py", "r")
# the content of the file object is read and used but not kept.
print(f.read()) 

print(f"""The second read() returns "".
The file content can be read only once.
print(f.read())
""")
print(f.read())
f.close() 

infile_2.py.txt:

import sys
#   Read a file specified by the first command line argument.
#   Primt the number of characters in the file and then the file itself.

if len(sys.argv) < 2:
    print("Usage: infile_2.py <filename>")
    sys.exit(1) # Exit with an error code

filename = sys.argv[1]

#   The default mode is "r" but explicitly specifying the mode as "r" provides clarity of intention.
f = open(filename, mode="r")
s = f.read()
print(f"Number of characters in the file: {len(s)}\n")
print(s)
f.close()

3.2 Closing Files

• Forget closing a file creates problems. Examples.
  • Data loss and data corruption.
  • Performance degradation.
  • Resource leakage.
• Thus, it is important to close a file when it is no longer used. Proper house keeping is very important in computer science.
• Use the with statement for context management.
• Format of the with statement:

with open(infile_name, 'r') as f:
      # block of code within the with statement
      # ...
# The file is automatically closed when exiting the with block,
# even if exceptions occur within the block.

Example:

infile_3.py.txt:

import sys
#   Read a file specified by the first command line argument.
#   Primt the number of characters in the file and then the file itself.

if len(sys.argv) < 2:
    print("Usage: infile_3.py <filename>")
    sys.exit(1) # Exit with an error code

filename = sys.argv[1]

#   The default mode is "r" but explicitly specifying the mode as "r" provides clarity of intention.
with open(filename, mode="r") as f:
   s = f.read()
   print(f"Number of characters in the file: {len(s)}\n")
   print(s)

3.3 I/O and File Exceptions

• Exceptions are runtime errors.
• If not handled, they terminate program execution.
• Common file exception examples:
  1. FileNotFoundError: cannot find file when reading.
  2. FileExistsError: file or directory already exists when trying to create them.
  3. PermissionError: such as writing to a read-only files.
  4. IOError: general I/O exceptions.
• The try-except blocks can be used to handle exceptions in Python.

Example:

infile_4.py.txt:

import sys
#   Read a file specified by the first command line argument.
#   Print the number of characters in the file and then the file itself.

if len(sys.argv) < 2:
    print("Usage: infile_4.py <filename>")
    sys.exit(1) # Exit with an error code

filename = sys.argv[1]

try:
    with open(filename, mode="r") as f:
        s = f.read()
        print(f"Number of characters in the file: {len(s)}\n")
        print(s)
except FileNotFoundError:
    print(f"The file {filename} cannot be found.")
    sys.exit(1)
except Exception as e:
    print(f"An error occurred: {e}"

Example:

infile_5.py.txt:

import sys
#   Read a file specified by the first command line argument.
#   Print the number of characters in the file and then the file itself.

if len(sys.argv) < 2:
    print("Usage: infile_4.py <filename>")
    sys.exit(1) # Exit with an error code

filename = sys.argv[1]

try:
    with open(filename, mode="r") as f:
        for i,line in enumerate(f,1):
            print(f"[{i}]: {line}", end='')
except FileNotFoundError:
    print(f"The file {filename} cannot be found.")
    sys.exit(1)
except Exception as e:
    print(f"An error occurred: {e}")
 

3.4 Some tips

• If you need to use os.path for portability (as suggested by Zybooks Chapter 9.3), you should instead consider using the more modern pathlib module.
• The open() method has many additional parameters for fine-tuning your need.

4. Output Files

• When writing to a file, the file mode can be set, such as:
  1. "w": writing to a file. Overwrite if the file already exists.
  2. "x": writing to a file. Fail if the file already exists.
  3. "a": append to the end of a file. Create the file is it does not already exists.

4.1 print()

• By now, you are probably familiar with the print() function.
• print() is not a method of a file object.
• Some properties of print()
  1. Use to output to stdout or files.
  2. Good for formatted output.
  3. can write 0 or many objects, which are converted to strings.
  4. Add a space as the separator between objects.
  5. Add a newline symbol at the end by default.
  6. Write to stdout by default.

Format:

print(*objects, sep=' ', end='\n', file=sys.stdout, flush=False)

1. *objects: 0 or more objects to output.
2. sep: separator between objects, default as ' '.
3. end: string to be add at the end, default as '\n'.
4. file: file to be written to, default as sys.stdout.
5. flush: whether the output is flushed and thus written to the file immediately, default to False.

4.2 write()

• write() is a method of a file object. It is thus called as f.write(), where f is a file object.
• It takes on only one argument.
• It is used for raw writing.
• No automatic \n added to the end.

Example:

output_1.py:

list_1 = ["Hello", "world", "it is me.", 2048]
print(f"list_1: {list_1}")

try:
    filename = 'output_1_out.txt'
    with open(filename, 'w') as f:
        print(f"list_1: {list_1}", file=f)
        #   Note that the int 2048 is converted to '2048' by print()
        print("Hello", "world", "it is me.", 2048, file=f)
        print("Hello", "world", "it is me.", 2048, sep=" :-) ", file=f)
        print("Hello", "world", "it is me.", 2048, sep=" :-) ", end='\nbye bye\n', file=f)
except FileExistsError:
    print(f"The file {filename} already exists.")
except Exception as e:
    print(f"An error occurred: {e}")

#   Append.
try:
    with open(filename, 'a') as f:
        f.write("\n\nThe following is written by write().")
        f.write("No newline symbol automatically added by write().\n")
        f.write(f"list_1: {list_1}")
# no FileExistsError for append mode.
except Exception as e:
    print(f"An error occurred: {e}")