String Conversion Tools

One of Python’s design mottos is that it refuses the temptation to guess. As a prime example, you cannot add a number and a string together in Python, even if the string looks like a number (i.e., is all digits):

>>> "42" + 1
TypeError: cannot concatenate 'str' and 'int' objects

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This is by design: because + can mean both addition and concatenation, the choice of conversion would be ambiguous. So, Python treats this as an error. In Python, magic is generally omitted if it will make your life more complex.

What to do, then, if your script obtains a number as a text string from a file or user interface? The trick is that you need to employ conversion tools before you can treat a string like a number, or vice versa. For instance:

>>> int("42"), str(42)          # Convert from/to string
(42, '42')
>>> repr(42)                    # Convert to as-code string
'42'

The int function converts a string to a number, and the str function converts a number to its string representation (essentially, what it looks like when printed). The repr function (and the older backquotes expression, removed in Python 3.0) also converts an object to its string representation, but returns the object as a string of code that can be rerun to recreate the object. For strings, the result has quotes around it if displayed with a print statement:

>>> print(str('spam'), repr('spam'))
('spam', "'spam'")

See the sidebar str and repr Display Formats for more on this topic. Of these, int and str are the generally prescribed conversion techniques.

Now, although you can’t mix strings and number types around operators such as +, you can manually convert operands before that operation if needed:

>>> S = "42"
>>> I = 1
>>> S + I
TypeError: cannot concatenate 'str' and 'int' objects

>>> int(S) + I            # Force addition
43

>>> S + str(I)            # Force concatenation
'421'

Similar built-in functions handle floating-point number conversions to and from strings:

>>> str(3.1415), float("1.5")
('3.1415', 1.5)

>>> text = "1.234E-10"
>>> float(text)
1.2340000000000001e-010

Later, we’ll further study the built-in eval function; it runs a string containing Python expression code and so can convert a string to any kind of object. The functions int and float convert only to numbers, but this restriction means they are usually faster (and more secure, because they do not accept arbitrary expression code). As we saw briefly in Chapter 5, the string formatting expression also provides a way to convert numbers to strings. We’ll discuss formatting further later in this chapter.

Character code conversions

On the subject of conversions, it is also possible to convert a single character to its underlying ASCII integer code by passing it to the built-in ord function—this returns the actual binary value of the corresponding byte in memory. The chr function performs the inverse operation, taking an ASCII integer code and converting it to the corresponding character:

>>> ord('s')
115
>>> chr(115)
's'

You can use a loop to apply these functions to all characters in a string. These tools can also be used to perform a sort of string-based math. To advance to the next character, for example, convert and do the math in integer:

>>> S = '5'
>>> S = chr(ord(S) + 1)
>>> S
'6'
>>> S = chr(ord(S) + 1)
>>> S
'7'

At least for single-character strings, this provides an alternative to using the built-in int function to convert from string to integer:

>>> int('5')
5
>>> ord('5') - ord('0')
5

Such conversions can be used in conjunction with looping statements, introduced in Chapter 4 and covered in depth in the next part of this book, to convert a string of binary digits to their corresponding integer values. Each time through the loop, multiply the current value by 2 and add the next digit’s integer value:

>>> B = '1101'                 # Convert binary digits to integer with ord
>>> I = 0
>>> while B != '':
...     I = I * 2 + (ord(B[0]) - ord('0'))
...     B = B[1:]
...
>>> I
13

A left-shift operation (I << 1) would have the same effect as multiplying by 2 here. We’ll leave this change as a suggested exercise, though, both because we haven’t studied loops in detail yet and because the int and bin built-ins we met in Chapter 5 handle binary conversion tasks for us in Python 2.6 and 3.0:

>>> int('1101', 2)             # Convert binary to integer: built-in
13
>>> bin(13)                    # Convert integer to binary
'0b1101'

Given enough time, Python tends to automate most common tasks!