Core Python Programming (2nd Edition)

14.3.2. compile()
compile() is a function that allows the programmer to generate a code object on the fly, that is, during
runtime. These objects can then be executed or evaluated using the exec statement or eval() BIF. It is
important to bring up the point that both exec and eval() can take string representations of Python code
to execute. When executing code given as strings, the process of byte-compiling such code must occur
every time. The compile() function provides a one-time byte-code compilation of code so that the
precompile does not have to take place with each invocation. Naturally, this is an advantage only if the
same pieces of code are executed more than once. In these cases, it is definitely better to precompile
the code.
All three arguments to compile() are required, with the first being a string representing the Python code
to compile. The second string, although required, is usually set to the empty string. This parameter
represents the file name (as a string) where this code object is located or can be found. Normal usage is
for compile() to generate a code object from a dynamically generated string of Python codecode that
obviously does not originate from an existing file.
The last argument is a string indicating the code object type. There are three possible values:
'eval' Evaluatable expression [to be used with eval()]
'single' Single executable statement [to be used with exec]
'exec' Group of executable statements [to be used with exec]
Evaluatable Expression
>>> eval_code = compile('100 + 200', '', 'eval')
>>> eval(eval_code)
300
Single Executable Statement
>>> single_code = compile('print"Hello world!"', '', 'single')
>>> single_code
>>> exec single_code
Hello world!
Group of Executable Statements
>>> exec_code = compile("""
... req = input('Count how many numbers? ')
... for eachNum in range(req):
... print eachNum
... """, '', 'exec')
>>> exec exec_code
Count how many numbers? 6
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