4.3. Input

4.3. Input
	Chapter 4. Communication with outside

As you can print results on the screen, you can read data from the keyboard which is the standard input device. Python provides the raw_input function for that, which is used as follows:

>>> nb = raw_input("Enter a number, please:")
Enter a number, please:12

The prompt argument is optional and the input has to be terminated by a return.

Important

	Important
`raw_input` always returns a string, even if you entered a number. Therefore you have to convert the string input by yourself into whatever you need. Table 4.3 gives an overview of all possible type conversion function. >>> nb '12' >>> type(nb) <type 'str'> >>> nb = int(nb) >>> nb 12 >>> type(nb) <type 'int'> Notice that a user can enter whatever he wants. So, the input is probably not what you want, and the type conversion can therefore fail. It is careful to test before converting input strings. >>> nb = raw_input("Please enter a number:") Please enter a number:toto >>> nb 'toto' >>> int(nb) Traceback (most recent call last): File "<stdin>", line 1, in ? ValueError: invalid literal for int(): toto The following function controls the input: def read_number(): while 1: nb = raw_input("Please enter a number:") try: nbconv = int(nb) except: print nb, "is not a number." continue else: break return nb and produces the following output: >>> read_number() Please enter a number:toto toto is not a number. Please enter a number:12 '12'

raw_input always returns a string, even if you entered a number. Therefore you have to convert the string input by yourself into whatever you need. Table 4.3 gives an overview of all possible type conversion function.

>>> nb
'12'
>>> type(nb)
<type 'str'>
>>> nb = int(nb)
>>> nb
12
>>> type(nb)
<type 'int'>

Notice that a user can enter whatever he wants. So, the input is probably not what you want, and the type conversion can therefore fail. It is careful to test before converting input strings.

>>> nb = raw_input("Please enter a number:")
Please enter a number:toto
>>> nb
'toto'
>>> int(nb)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: invalid literal for int(): toto

The following function controls the input:

def read_number():
    while 1:
        nb = raw_input("Please enter a number:")
        try:
            nbconv = int(nb)
        except:
            print nb, "is not a number."
            continue
        else:
            break
    return nb

and produces the following output:

>>> read_number()
Please enter a number:toto
toto is not a number.
Please enter a number:12
'12'

Table 4.3. Type conversion functions

Function	Description
`int(x [,base])`	converts `x` to an integer
`long(x [,base])`	converts `x` to a long integer
`float(x)`	converts `x` to a floating-point number
`complex(real [,imag])`	creates a complex number
`str(x)`	converts `x` to a string representation
`repr(x)`	converts `x` to an expression string
`eval(str)`	evaluates `str` and returns an object
`tuple(s)`	converts a sequence object to a tuple
`list(s)`	converts a sequence object to a list
`chr(x)`	converts an integer to a character
`unichr(x)`	converts an integer to a Unicode character
`ord(c)`	converts a character to its integer value
`hex(x)`	converts an integer to a hexadecimal string
`oct(x)`	converts an integer to an octal string


4.2. Formatting strings		4.4. Files