python_eval_exec_examples.py

# Basic examples
>>> var1 = 100
>>> eval("50 + var1")
150
>>> eval("var1 = 25")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>  
  File "<string>", line 1
    var1 = 25
         ^
SyntaxError: invalid syntax
>>> exec("var1 = 25") # returns None
>>> var1
25

# compile() examples
>>> exec(compile("var1 = 125", "<string>", "exec")) # returns None
>>> var1
125
>>> eval(compile("75", "<string>", "eval"))
75
>>> exec(compile("75", "<string>", "eval")) # code object (generated by compile(), returns 75, but ignored by exec), so returns None
>>> eval(compile("75", "<string>", "exec")) # compile returns None, passed through by eval
>>> eval(compile("75", "<string>", "exec")) == None
True
>>> from dis import dis
>>> dis(compile("var1 = 125", "<string>", "exec"))
  1           0 LOAD_CONST               0 (125)
              3 STORE_NAME               0 (var1)
              6 LOAD_CONST               1 (None) # here we see that None is returned
              9 RETURN_VALUE

# A loop example with exec()
>>> exec("""for i in range(0,5):
...  print('Test number: %s' % (i))""")
Test number: 0
Test number: 1
Test number: 2
Test number: 3
Test number: 4

# Example with usage of a declared method
>>> def example_method(parameter):
...  print("You gave parameter: %s" % (parameter))
...  return parameter + " rules!"
...
>>> var1 = example_method('Python') # This is an expression, so it both eval as exec should be able to process it
You gave parameter: Python
>>> exec("example_method('Python')")
You gave parameter: Python
>>> eval("example_method('Python')")
You gave parameter: Python
'Python rules!'

# Example with the usage of a while loop
>>> exec("""i = 0
... while i < 3:
...   print('Test number: %s' % (i))
...   i += 1""")
Test number: 0
Test number: 1
Test number: 2
>>> eval("""i = 0
... while i < 3:
...  print('Test number: %s' % (i))
...  i += 1""")
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
  File "<string>", line 1
    i = 0
      ^
SyntaxError: invalid syntax

# Example of globals/locals to retrieve variable declared within exec()
>>> global_vars = dict()
>>> local_vars = dict()
>>> exec("global global_var1; global_var1, local_var1 = 100, 50", global_vars, local_vars)
>>> global_vars["global_var1"]
100
>>> local_vars["local_var1"]
50
>>> global_var1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'global_var1' is not defined
>>> local_var1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'local_var1' is not defined

# Timeit time!
$ python -m timeit -s 'var1 = 100; var2 = 50; var1 * var2'
100000000 loops, best of 3: 0.00785 usec per loop
$ python -m timeit -s "code_to_exec_compiled = compile('var1 = 100; var2 = 50; var1 * var2', '<string>', 'exec')" "exec(code_to_exec_compiled)"
1000000 loops, best of 3: 0.425 usec per loop
$ python -m timeit -s "code_to_exec = 'var1 = 100; var2 = 50; var1 * var2'" "exec(code_to_exec)"
100000 loops, best of 3: 9.33 usec per loop