Programmer's guide to python

9.1 I/O handling functions

There are three built-in functions for I/O handling in Python, let's go through them one by one.

9.1.1 input(prompt) => None

Parameters:

>prompt Any: Your message to the screen.

Explanation: Reads input from standard input devices (such as keyboard) and takes it input as string.

## Take input from user
v = input() # or "input('Your message here ')"
print(type(v)) # <class 'str'>
print(v)

9.1.2 print(*values, sep=' ', end='\n', file=sys.stdout, flush=False) => None

Parameters:

>values object: Takes the object to be printed, * indicates more than one object.
>sep Optional[str]: A separator between multiple values, default is a whitespace.
>end Optional[str]: The last print value, default is a newline.
>file Optional[Writer]: A file-like object (stream), default is screen.
>flush bool: Whether to forcibly flush the stream.

Explanation: Prints the given object to a standard output device, usually screen.

## Print output to the screen
print("This is output")  # This is output
print("multiple", "objects", "to print")  # multiple objects to print


## Pass variables to print
var1 = var2 = 42
print("The value of var is:", var1)  # The value of var is: 42
print("var is", var1, "right?")  # var is 42 right?
# format string with numbers are more readable
print("vars are {0} and {1}".format(var1, var2))  # vars are 42 and 42
# or simply using f shorthand as it is more convenient
print(f"vars are {var1} and {var2}")  # vars are 42 and 42


## 'sep' and 'end' parameters 
# Use the 'sep' parameter on prints
print("This", "is", "a", "String", sep="_")  # This_is_a_String

# Use 'end' parameter to change print's end value
print("This", end="\t")
print("is not new line")  # This    is not new line

9.1.3 open(filename, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None) => file

Parameters:

>filename Union[str, bytes]: String or byte path to a file.
>mode str: Opening file mode.
>buffering int: For specifying buffering policy.
>encoding Optional[str]: Specify encoding format. On Windows the default is "cp1252" and "utf-8" on Linux/OSX.
>errors Optional[str]: To handle Encoding/Decoding errors.
>newline Optional[str]: Specify how the newline works.
>closefd bool: If set to False, the underlying file descriptor will be kept open even when the file is closed.
>opener Optional[Callable[[str, int], int]]: A custom file opener, it should return a file descriptor.

Explanation: This function is used to open a file. It returns a file object (also called handle), this object is further used to perform operations such as read/write/append to a file. For different types of operations there are different modes available, default is "r" (reading).

>
Various file modes are shown below.
1. >r: For opening a file in read-only mode.
2. >w: Opens a file in write mode, if the file already exists its previous content completely overwritten, else a new file is created and content can be written to it.
3. >a: In append mode if the file already exists and has some previous content, the new input is appended at the end and not overwritten, if it does not exist a new file is created and content can be written to it.
4. >x: In exclusive creation mode if the file already exists, the operation fails, else a new file is created and content can be written to it.
5. >t: Opening a file in text format, reading a file data as strings, this is the default format.
6. >b: Opening a file in binary format, reading a file data as bytes, this is used to handle non-text files like images/database/documents.

## Write to a file, 'PermissionError' is raised if Python doesn't have
# the permission
file = open("sample.txt", mode="w")
file.write("This is some text written to this file.")
file.close()


## Append mode
file = open("sample.txt", mode="a")
file.write("Previous content will not be overwritten.")
file.close()


## Read from a file, 'FileNotFoundError' is raised if the file is not found
file = open("sample.txt", mode="r")
# or open("sample.txt")

print(file.read())  # This is some text written to this file. \
                  # Previous content will not be overwritten.
file.close()


## Best practice is to write 'open()' in a 'try..except' block
try:
  file = open("sample.txt", mode="r")
  # do something
except FileNotFoundError:
  print("The file does not exist")
finally:
file.close()

>Mixing modes, you can't mix "a","r" and "w" modes together, but can mix other modes.

## Example 1: Text format and write mode
file = open("sample.txt", mode="tw")
file.write("This is some text written to the file")
file.close()


## Example 2: Read and append mode
file = open("sample.txt", mode="r+")
print(file.read()) # This is some text written to the file
file.write("Appending some more text in here.")
file.close()


## Example 3: Binary format and read mode, create a 'some.py' in
# current directory
file = open("some.py", mode="br")
print(file.read())
file.close()

>Various file object methods.

## Reading
# read all text/non-text data at once, returns string/bytes
file = open("sample.txt", mode="r")
print(file.read())

# read all text/non-text data line by line, returns string/byte list
print(file.readlines())


## Writing
# write single string to the file
file.write("This is some text.")

# take a iterable object containing multiple strings and writes to the file
file.writelines(["This is a text.", "This is also some text"])
# close a file, frees up system resources, should be called as the last step
file.close()