What is a method?
A method is a function on some object x that you normally call as x.name(arguments...). Methods are defined as functions inside the class definition: class C: def meth (self, arg): return arg*2 + self.attribute
How can I find the methods or attributes of an object?
For an instance x of a user-defined class, dir(x) returns an alphabetized list of the names containing the instance attributes and methods and attributes defined by its class.
How do I call a method defined in a base class from a derived class that overrides it?
If you're using new-style classes, use the built-in super() function: class Derived(Base): def meth (self): super(Derived, self).meth() If you're using classic classes: For a class definition such as class Derived(Base): ... you can call method meth() defined in Base (or one of Base's base classes) as Base.meth(self, arguments...). Here, Base.meth is an unbound method, so you need to provide the self argument.
How can I organize my code to make it easier to change the base class?
You could define an alias for the base class, assign the real base class to it before your class definition, and use the alias throughout your class. Then all you have to change is the value assigned to the alias. Incidentally, this trick is also handy if you want to decide dynamically (e.g. depending on availability of resources) which base class to use. Example: BaseAlias =class Derived(BaseAlias): def meth(self): BaseAlias.meth(self)
How do I find the current module name?
A module can find out its own module name by looking at the predefined global variable __name__. If this has the value '__main__', the program is running as a script. Many modules that are usually used by importing them also provide a command-line interface or a self-test, and only execute this code after checking __name__: def main(): print 'Running test...' ... if __name__ == '__main__': main() __import__('x.y.z') returns Try: __import__('x.y.z').y.z For more realistic situations, you may have to do something like m = __import__(s) for i in s.split(".")[1:]: m = getattr(m, i)
How do I copy a file?
The shutil module contains a copyfile() function.
You can get a pointer to the module object as follows: module = PyImport_ImportModule(""); If the module hasn't been imported yet (i.e. it is not yet present in sys.modules), this initializes the module; otherwise it simply returns the value of sys.modules[""]. Note that it doesn't enter the module into any namespace -- it only ensures it has been initialized and is stored in sys.modules. You can then access the module's attributes (i.e. any name defined in the module) as follows: attr = PyObject_GetAttrString(module, ""); Calling PyObject_SetAttrString() to assign to variables in the module also works.
How do I interface to C++ objects from Python?
Depending on your requirements, there are many approaches. To do this manually, begin by reading the "Extending and Embedding" document. Realize that for the Python run-time system, there isn't a whole lot of difference between C and C++ -- so the strategy of building a new Python type around a C structure (pointer) type will also work for C++ objects.
Collect the arguments using the * and ** specifier in the function's parameter list; this gives you the positional arguments as a tuple and the keyword arguments as a dictionary. You can then pass these arguments when calling another function by using * and **: def f(x, *tup, **kwargs): ... kwargs['width']='14.3c' ... g(x, *tup, **kwargs) In the unlikely case that you care about Python versions older than 2.0, use 'apply': def f(x, *tup, **kwargs): ... kwargs['width']='14.3c' ... apply(g, (x,)+tup, kwargs)
How do you make a higher order function in Python?
You have two choices: you can use nested scopes or you can use callable objects. For example, suppose you wanted to define linear(a,b) which returns a function f(x) that computes the value a*x+b. Using nested scopes: def linear(a,b): def result(x): return a*x + b return result Or using a callable object: class linear: def __init__(self, a, b): self.a, self.b = a,b def __call__(self, x): return self.a * x + self.b In both cases: taxes = linear(0.3,2) gives a callable object where taxes(10e6) == 0.3 * 10e6 + 2. The callable object approach has the disadvantage that it is a bit slower and results in slightly longer code. However, note that a collection of callables can share their signature via inheritance: class exponential(linear): # __init__ inherited def __call__(self, x): return self.a * (x ** self.b) Object can encapsulate state for several methods: class counter: value = 0 def set(self, x): self.value = x def up(self): self.value=self.value+1 def down(self): self.value=self.value-1 count = counter() inc, dec, reset = count.up, count.down, count.set Here inc(), dec() and reset() act like functions which share the same counting variable.
How do I convert a number to a string?
To convert, e.g., the number 144 to the string '144', use the built-in function str(). If you want a hexadecimal or octal representation, use the built-in functions hex() or oct(). For fancy formatting, use the % operator on strings, e.g. "%04d" % 144 yields '0144' and "%.3f" % (1/3.0) yields '0.333'. See the library reference manual for details.
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A method is a function on some object x that you normally call as x.name(arguments...). Methods are defined as functions inside the class definition: class C: def meth (self, arg): return arg*2 + self.attribute
How can I find the methods or attributes of an object?
For an instance x of a user-defined class, dir(x) returns an alphabetized list of the names containing the instance attributes and methods and attributes defined by its class.
How do I call a method defined in a base class from a derived class that overrides it?
If you're using new-style classes, use the built-in super() function: class Derived(Base): def meth (self): super(Derived, self).meth() If you're using classic classes: For a class definition such as class Derived(Base): ... you can call method meth() defined in Base (or one of Base's base classes) as Base.meth(self, arguments...). Here, Base.meth is an unbound method, so you need to provide the self argument.
How can I organize my code to make it easier to change the base class?
You could define an alias for the base class, assign the real base class to it before your class definition, and use the alias throughout your class. Then all you have to change is the value assigned to the alias. Incidentally, this trick is also handy if you want to decide dynamically (e.g. depending on availability of resources) which base class to use. Example: BaseAlias =
A module can find out its own module name by looking at the predefined global variable __name__. If this has the value '__main__', the program is running as a script. Many modules that are usually used by importing them also provide a command-line interface or a self-test, and only execute this code after checking __name__: def main(): print 'Running test...' ... if __name__ == '__main__': main() __import__('x.y.z') returns Try: __import__('x.y.z').y.z For more realistic situations, you may have to do something like m = __import__(s) for i in s.split(".")[1:]: m = getattr(m, i)
The shutil module contains a copyfile() function.
You can get a pointer to the module object as follows: module = PyImport_ImportModule("
How do I interface to C++ objects from Python?
Depending on your requirements, there are many approaches. To do this manually, begin by reading the "Extending and Embedding" document. Realize that for the Python run-time system, there isn't a whole lot of difference between C and C++ -- so the strategy of building a new Python type around a C structure (pointer) type will also work for C++ objects.
Collect the arguments using the * and ** specifier in the function's parameter list; this gives you the positional arguments as a tuple and the keyword arguments as a dictionary. You can then pass these arguments when calling another function by using * and **: def f(x, *tup, **kwargs): ... kwargs['width']='14.3c' ... g(x, *tup, **kwargs) In the unlikely case that you care about Python versions older than 2.0, use 'apply': def f(x, *tup, **kwargs): ... kwargs['width']='14.3c' ... apply(g, (x,)+tup, kwargs)
How do you make a higher order function in Python?
You have two choices: you can use nested scopes or you can use callable objects. For example, suppose you wanted to define linear(a,b) which returns a function f(x) that computes the value a*x+b. Using nested scopes: def linear(a,b): def result(x): return a*x + b return result Or using a callable object: class linear: def __init__(self, a, b): self.a, self.b = a,b def __call__(self, x): return self.a * x + self.b In both cases: taxes = linear(0.3,2) gives a callable object where taxes(10e6) == 0.3 * 10e6 + 2. The callable object approach has the disadvantage that it is a bit slower and results in slightly longer code. However, note that a collection of callables can share their signature via inheritance: class exponential(linear): # __init__ inherited def __call__(self, x): return self.a * (x ** self.b) Object can encapsulate state for several methods: class counter: value = 0 def set(self, x): self.value = x def up(self): self.value=self.value+1 def down(self): self.value=self.value-1 count = counter() inc, dec, reset = count.up, count.down, count.set Here inc(), dec() and reset() act like functions which share the same counting variable.
How do I convert a number to a string?
To convert, e.g., the number 144 to the string '144', use the built-in function str(). If you want a hexadecimal or octal representation, use the built-in functions hex() or oct(). For fancy formatting, use the % operator on strings, e.g. "%04d" % 144 yields '0144' and "%.3f" % (1/3.0) yields '0.333'. See the library reference manual for details.