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Python Crash Course Functions, Modules Bachelors V1.0 dd 20-01-2014 Hour 1 Introduction to language - functions What are functions A function is a piece of code in a program. The function performs a specific task. The advantages of using functions are: • Reducing duplication of code • Decomposing complex problems into simpler pieces • Improving clarity of the code • Reuse of code • Information hiding Functions in Python are first-class citizens. It means that functions have equal status with other objects in Python. Functions can be assigned to variables, stored in collections or passed as arguments. This brings additional flexibility to the language. Function types There are two basic types of functions. Built-in functions and user defined ones. The built-in functions are part of the Python language. Examples are: dir(), len() or abs(). Introduction to language - functions Defining Functions Here are simple rules to define a function in Python: • Function blocks begin with the keyword def followed by the function name and parentheses ( ). • Any input parameters or arguments should be placed within these parentheses. You can also define parameters inside these parentheses. • The code block within every function starts with a colon : and is indented. • The statement return [expression] exits a function, optionally passing back an expression to the caller. A return statement with no arguments is the same as return None. >>> def my_func(x, y, z): ... a = x + y ... b = a * z ... return b ... >>> >>> 8.0 >>> 4.0 >>> 5.0 >>> 6.0 my_func(1.0, 3.0, 2.0) my_func(1.0, 3.0, 1.0) my_func(5.0, 0.0, 1.0) my_func(2.0, 0,0 3.0) Introduction to language - functions Defining Functions Function must be denifed preceding their usage: where to define functions #!/usr/bin/python #!/usr/bin/python def f1(): print "f1()" def f(): print "f() function" f1() #f2() def g(): def f(): print "f() inner function" f() def f2(): print "f2()" f() g() uncommenting f2() will cause a NameError inner function definition Introduction to language - functions >>> def f(): ... pass ... >>> def f(): >>> def g(): ... """This function prints a message ... """ pass ... print "Today it is a cloudy day" ... ... >>> def h(f): >>> f.__doc__ ... print id(f) 'This function prints a message ' ... >>> f() >>> a=(f, g, h) Today it is a cloudy day >>> for i in a: >>> id(f) ... print i 140491806602016 ... >>> <function f at 0x7fc6cc39f320> <function g at 0x7fc6cc39f398> <function h at 0x7fc6caab1c80> >>> h(f) 140491806602016 >>> h(g) 140491806602136 >>> • Functions are objects Introduction to language - functions Functions types • • • always available for usage those contained in external modules programmer defined >>> >>> ... ... >>> 1 >>> 729 >>> 9.0 from math import sqrt def cube(x): return x * x * x print abs(-1) print cube(9) print sqrt(81) Introduction to language - functions The return keyword • • is used to return value no return returns None >>> >>> ... ... ... ... ... ... ... >>> >>> (5, >>> >>> 5 1 n = [1, 2, 3, 4, 5] def stats(x): mx = max(x) mn = min(x) ln = len(x) sm = sum(x) return mx, mn, ln, sm mx, mn, ln, sm = stats(n) print stats(n) 1, 5, 15) print mx, mn, ln, sm 5 15 >>> def cube(x): ... return x * x * x ... >>> def showMessage(msg): ... print msg ... >>> x = cube(3) >>> print x 27 >>> showMessage("Some text") Some text >>> print showMessage("O, no!") O, no! None >>> showMessage(cube(3)) 27 >>> Introduction to language - functions >>> def fact(n): ... if(n==0): return 1; ... m = 1; ... k = 1; ... while(n >= k): ... m = m * k; ... k = k + 1; ... return m; Recursion: >>> def fact(n): ... if n > 0: ... return n * fact(n-1) ... return 1 >>> print fact(100) >>> print fact(1000) # Recursive call # exits function returning 1 Introduction to language - functions Function arguments single arguments >>> ... ... >>> >>> 212 >>> 32 >>> 86 >>> def C2F(c): return c * 9/5 + 32 print C2F(100) print C2F(0) print C2F(30) multiple arguments >>> def power(x, y=2): ... r = 1 ... for i in range(y): ... r = r * x ... return r ... >>> print power(3) 9 >>> print power(3, 3) 27 >>> print power(5, 5) 3125 >>> Introduction to language - functions Function arguments named arguments • • order may be changed default value >>> def display(name, age, sex): ... print "Name: ", name ... print "Age: ", age ... print "Sex: ", sex ... >>> display(age=43, name="Lary", sex="M") Name: Lary Age: 43 Sex: M >>> display(name="Joan", age=24, sex="F") Name: Joan Age: 24 Sex: F >>> display("Joan", sex="F", age=24) Name: Joan Age: 24 Sex: F >>> display(age=24, name="Joan", "F") File "<stdin>", line 1 SyntaxError: non-keyword arg after keyword arg >>> Introduction to language - functions Function arguments arbitrary number of arguments >>> def sum(*args): ... '''Function returns the sum ... of all values''' ... s = 0 ... for i in args: ... s += i ... return s ... >>> >>> print sum.__doc__ Function returns the sum of all values >>> print sum(1, 2, 3) 6 >>> print sum(1, 2, 3, 4, 5) 15 >>> Introduction to language - functions Function arguments passing by reference Passing objects by reference has two important conclusions. The process is faster than if copies of objects were passed. Mutable objects that are modified in functions are permanently changed. >>> n = [1, 2, 3, 4, 5] >>> >>> print "Original list:", n Original list: [1, 2, 3, 4, 5] >>> >>> def f(x): ... x.pop() ... x.pop() ... x.insert(0, 0) ... print "Inside f():", x ... ... >>> f(n) Inside f(): [0, 1, 2, 3] >>> >>> print "After function call:", n After function call: [0, 1, 2, 3] >>> Introduction to language - functions Function variables >>> name = "Jack" >>> def f(): ... name = "Robert" >>> name = "Jack" ... print "Within function", name A variable defined in af(): function >>> def ... body has a local ... scope global name >>> print "Outside function", name ... name = "Robert" Outside function Jack ...contents print name We can get the of a"Within global function", >>> f() ... variable inside the body of a Within function Robert name function. But >>> if weprint want to"Outside change afunction", >>> def f(): Outside function global variable in a function, we Jack ... print "Within function", name must use the>>> global keyword. f() ... Within function Robert >>> print "Outside function", name >>> print "Outside function", name Outside function Jack Outside function Robert >>> f() >>> Within function Jack >>> Global and Local Introduction to language - functions The Anonymous Functions: You can use the lambda keyword to create small anonymous functions. These functions are called anonymous because they are not declared by using the def keyword. • Lambda forms can take any number of arguments but return just one value in the form of an expression. They cannot contain commands or multiple expressions. • An anonymous function cannot be a direct call to print because lambda requires an expression. • Lambda functions have their own local namespace and cannot access variables other than those in their parameter list and those in the global namespace. #!/usr/bin/python # Function definition is here sum = lambda arg1, arg2: arg1 + arg2; # Now you can call sum as a function print "Value of total : ", sum( 10, 20 ) print "Value of total : ", sum( 20, 20 ) Value of total : 30 Value of total : 40 Introduction to languge - Modules What are modules for? Python modules are used to organize Python code. For example, database related code is placed inside a database module, security code in a security module etc. Smaller Python scripts can have one module. But larger programs are split into several modules. Modules are grouped together to form packages. Modules names A module name is the file name with the .py extension. When we have a file called empty.py, empty is the module name. The __name__ is a variable that holds the name of the module being referenced. The current module, the module being executed (called also the main module) has a special name: '__main__'. With this name it can be referenced from the Python code. Introduction to language - Modules $ cat hello.py def print_func( par ): print "Hello : ", par return #!/usr/bin/python # Import module hello import hello # Now you can call defined function that module as follows hello.print_func(“Earth") Hello : Earth >>> print __name__ __main__ >>> print hello.__name__ hello >>> Importing into the current namespace should be done with care due to name clashes Introduction to languge - Modules When you import a module, the Python interpreter searches for the module in the following sequences: • The current directory. • If the module isn't found, Python then searches each directory in the shell variable PYTHONPATH. • If all else fails, Python checks the default path. On UNIX, this default path is normally /usr/lib64/python2.7/. The module search path is stored in the system module sys as the sys.path variable. The sys.path variable contains the current directory, PYTHONPATH, and the installationdependent default. PYTHONPATH is an environment variable, consisting of a list of directories. The syntax of PYTHONPATH is the same as that of the shell variable PATH. /software/local/lib64/python2.7/site-packages /usr/lib64/python2.7/site-packages Introduction to language - modules Modules are searched for in the following places: • the current working directory (for interactive sessions) • the directory of the top-level script file (for script files) • the directories defined in PYTHONPATH • Standard library directories >>> # Get the complete module search path: >>> import sys >>> print sys.path ['', '/software/local/lib64/python2.7/site-packages/Astropysics-0.1.dev_r1161py2.7.egg', '/software/local/lib64/python2.7/site-packages/CosmoloPy-0.1.104-py2.7linux-x86_64.egg', '/software/local/lib64/python2.7/site-packages/pyregion-1.1_gitpy2.7-linux-x86_64.egg', '/software/local/lib64/python2.7/site-packages/scikit_image0.9dev-py2.7-linux-x86_64.egg', '/software/local/lib64/python2.7/sitepackages/memory_profiler-0.26-py2.7.egg', '/software/local/lib64/python2.7/sitepackages/agpy-0.1.1-py2.7.egg', '/software/local/lib64/python2.7/site-packages/APLpy0.9.12-py2.7.egg', '/software/local/lib64/python2.7/site-packages/pandas-0.14.1py2.7-linux-x86_64.egg', '/software/local/lib64/python2.7/site-packages/astroquery0.2.3-py2.7.egg', '/software/local/lib64/python2.7/site-packages/html5lib-1.0b3- Introduction to language - modules Frequently used modules • • • • • • • • • • sys Information about Python itself (path, etc.) os Operating system functions os.path Portable pathname tools shutil Utilities for copying files and directory trees cmp Utilities for comparing files and directories glob Finds files matching wildcard pattern re Regular expression string matching time Time and date handling datetime Fast implementation of date and time handling doctest, unittest Modules that facilitate unit test Introduction to language - modules More frequently used modules • pdb Debugger • hotshot Code profiling • pickle, cpickle, marshal, shelve Used to save objects and code to files • getopt, optparse Utilities to handle shell-level argument parsing • math, cmath Math functions (real and complex) faster for scalars • random Random generators (likewise) • gzip read and write gzipped files • struct Functions to pack and unpack binary data structures • StringIO, cStringIO String-like objects that can be read and written as files (e.g., in-memory files) • types Names for all the standard Python type Introduction to language - modules • Modules can contain any code • Classes, functions, definitions, immediately executed code • Can be imported in own namespace, or into the global namespace >>> import math >>> math.cos(math.pi) -1.0 >>> math.cos(pi) Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'pi' is not defined >>> from math import cos, pi >>> cos(pi) -1.0 >>> from math import * Introduction to language - modules Module import >>> from math import * This construct will import all Python definitions into the namespace of another module. he use of this import construct may result in namespace pollution. We may have several objects of the same name and their definitions can be overridden. #!/usr/bin/python """ names is a test module """ _version = 1.0 names = ["Paul", "Frank", "Jessica"] def show_names(): for i in names: print i def _show_version(): print _version >>> from names import * >>> print locals() {'__builtins__': <module '__builtin__' (built-in)>, '__file__': './private.py', 'show_names': <function show_names at 0xb7dd233c>, 'names': ['Paul', 'Frank', 'Jessica'], '__name__': '__main__', '__doc__': None} >>> show_names() Paul Frank Jessica No _ names are imported Introduction to language - modules >>> from math import >>> print sin(1.0) >>> print cos(1.0) # >>> from math import >>> # All attributes Extremely dangerous >>> print tan(1.0) sin won’t work * copied to global namespace • Use from...import and import...as with care. Both make your code harder to understand. • Do not sacrifice code clearness for some keystrokes! • In some cases, the use is acceptable: – In interactive work (import math as m) – If things are absolutely clear (e.g. all functions of an imported module obey a clear naming convention; cfits_xyz) import.. as: As last resort in case of name clashes between module names Introduction to language - modules • Inspecting module methods >>> import numpy >>> dir(numpy) ['ALLOW_THREADS', 'BUFSIZE', 'CLIP', 'ComplexWarning', 'DataSource', 'ERR_CALL', 'ERR_DEFAULT', 'ERR_DEFAULT2', 'ERR_IGNORE', 'ERR_LOG', 'ERR_PRINT', 'ERR_RAISE', 'ERR_WARN', 'FLOATING_POINT_SUPPORT', 'FPE_DIVIDEBYZERO', 'FPE_INVALID', 'FPE_OVERFLOW', 'FPE_UNDERFLOW', 'False_', 'Inf', 'Infinity', 'MAXDIMS', 'MachAr', 'NAN', 'NINF', 'NZERO', 'NaN', 'PINF', 'PZERO', 'PackageLoader', 'RAISE', 'RankWarning', 'SHIFT_DIVIDEBYZERO', 'SHIFT_INVALID', 'SHIFT_OVERFLOW', 'SHIFT_UNDERFLOW', 'ScalarType', 'Tester', 'True_', 'UFUNC_BUFSIZE_DEFAULT', 'UFUNC_PYVALS_NAME', 'WRAP', '__NUMPY_SETUP__', '__all__', '__builtins__', '__config__', '__doc__', '__file__', '__git_revision__', '__name__', '__package__', '__path__', '__version__', '_import_tools', '_mat', 'abs', 'absolute', 'add', 'add_docstring', 'add_newdoc', 'add_newdocs', 'alen', 'all', 'allclose', 'alltrue', 'alterdot', 'amax', 'amin', 'angle', 'any', 'append', 'apply_along_axis', ... 'typeNA', 'typecodes', 'typename', 'ubyte', 'ufunc', 'uint', 'uint0', 'uint16', 'uint32', 'uint64', 'uint8', 'uintc', 'uintp', 'ulonglong', 'unicode', 'unicode0', 'unicode_', 'union1d', 'unique', 'unpackbits', 'unravel_index', 'unsignedinteger', 'unwrap', 'ushort', 'vander', 'var', 'vdot', 'vectorize', 'version', 'void', 'void0', 'vsplit', 'vstack', 'where', 'who', 'zeros', 'zeros_like'] Introduction to language - modules Executing modules Modules can be imported into other modules or they can be also executed. Module authors often create a testing suite to test the module. Only if the module is executed as a script, the __name__ attribute equals to __main__. #!/usr/bin/python """ A module containing the fibonacci function. """ def fib(n): a, b = 0, 1 while b < n: print b, (a, b) = (b, a + b) # testing if __name__ == '__main__': fib(500) $ ./fibonacci.py 1 1 2 3 5 8 13 21 34 55 89 144 233 377 Introduction to language - modules • Importing submodules >>> import numpy >>> numpy.random # Submodule >>> numpy.random.randn() # Function in submodule ---------------------------------- (Restart Python) >>> import numpy.random # Import submodule only >>> numpy.random.randn() ---------------------------------- (Restart Python) >>> from numpy import random # Alternative form >>> random.randn() ---------------------------------- (Restart Python) >>> from numpy.random import * # Previous warnings >>> randn() # apply here as well! Your own package The main difference between a module and a package is that a package is a collection of modules AND it has an __init__.py file. myMath/ __init__.py adv/ __init__.py sqrt.py add.py subtract.py multiply.py divide.py # add.py # sqrt.py def add(x, y): import math """""" return x + y def squareroot(n): """""" return math.sqrt(n) # outer __init__.py from add import add from divide import division from multiply import multiply from subtract import subtract from adv.sqrt import squareroot import mymath print print print print mymath.add(4,5) mymath.division(4, 2) mymath.multiply(10, 5) mymath.squareroot(48)) Introduction to language End