This notebook was put together by Jake Vanderplas for UW's Astro 599 course. Source and license info is on GitHub.
An important part of coding (in Python and in other modern language) is organizing code in easily re-used chunks.
Python can work within both a procedural and an object-oriented style.
Procedural programming is using functions
Object-oriented programming is using classes
We'll come back to classes later, and look at functions now.
Function definitions in Python look like this:
def function_name(arg1, arg2, ...,
kw1=val1, kw2=val2, ...)
(Note that line-breaks between the parentheses are ignored)
argX are arguments, and are required
kwX are keyword arguments, and are optional
The function name can be anything, as long as it:
print
, or for
)Note for IDL users: there is no difference between functions and procedures. All Python functions return a value: if no return is specified, it returns None
def addnums(x, y):
return x + y
result = addnums(1, 2)
print result
3
print addnums(1, y=2)
3
print addnums("A", "B")
AB
Note that the variable types are not declared (as we've discussed Python is a dynamic language)
def scale(x, factor=2.0):
return x * factor
scale(4)
8.0
scale(4, 10)
40
scale(4, factor=10)
40
Arguments and Keyword arguments can either be specified by order or by name, but an unnamed argument cannot come after a named argument:
scale(x=4, 10)
File "<ipython-input-13-f0cbe9ee0750>", line 1 scale(x=4, 10) SyntaxError: non-keyword arg after keyword arg
Returned values can be anything, which allows a lot of flexibility:
def build_dict(x, y):
return {'x':x, 'y':y}
build_dict(4, 5)
{'x': 4, 'y': 5}
def no_return_value():
pass
x = no_return_value()
print x
None
Keyword arguments can be a very handy way to grow new functionality without breaking old code.
Imagine, for example, you had the build_dict
function from above:
def build_dict(x, y):
return {'x':x, 'y':y}
build_dict(1, 2)
{'x': 1, 'y': 2}
Now what if you want to change the names of the variables in the dictionary? Adding a keyword argument can allow this flexibility without breaking old code:
def build_dict(x, y, xname='x', yname='y'):
return {xname:x, yname:y}
build_dict(1, 2) # old call still works
{'y': 2, 'x': 1}
build_dict(1, 2, xname='spam', yname='eggs')
{'eggs': 2, 'spam': 1}
This is admittedly a silly example, but it shows how keywords can be used to add flexibility without breaking old APIs.
Python functions have their own local variables list:
def modify_x(x):
x += 5
return x
x = 10
y = modify_x(x)
print x
print y
10 15
Modifying a variable in the function does not modify the variable globally... unless you use the global
declaration
def add_a(x):
global a
a += 1
return x + a
a = 10
print add_a(5)
print a
16 11
Warning: Simple and Compound types are treated differently!
def add_one(x):
x += 1
x = 4
add_one(x)
print x
4
def add_element(L):
L.append(4)
L = [1, 2]
add_element(L)
print L
[1, 2, 4]
Simple types (int, long, float, complex, string) are passed by value.
Compound types (list, dict, set, tuple, user-defined objects) are passed by reference.
Question to think about: why would this be?
def cheeseshop(kind, *args, **kwargs):
print "Do you have any", kind, "?"
print "I'm sorry, we're all out of", kind
for arg in args:
print arg
print 40 * "="
for kw in kwargs:
print kw, ":", kwargs[kw]
cheeseshop("Limburger", "It's very runny, sir.",
"It's really very, VERY runny, sir.",
shopkeeper="Michael Palin",
client="John Cleese",
sketch="Cheese Shop Sketch")
Do you have any Limburger ? I'm sorry, we're all out of Limburger It's very runny, sir. It's really very, VERY runny, sir. ======================================== shopkeeper : Michael Palin sketch : Cheese Shop Sketch client : John Cleese
(example from Python docs)
Documentation is not required, but your future self (and anybody else using your code) will thank you.
def power_of_difference(x, y, p=2.0):
"""Return the power of the difference of x and y
Parameters
----------
x, y : float
the values to be differenced
p : float (optional)
the exponent (default = 2.0)
Returns
-------
result: float
(x - y) ** p
"""
diff = x - y
return diff ** p
power_of_difference(10.0, 5.0)
25
(Note that this example follows the Numpy documentation standard)
With documentation specified this way, the IPython help
command will be helpful!
power_of_difference?
%%file myfile.py
def power_of_difference(x, y, p=2.0):
"""Return the power of the difference of x and y
Parameters
----------
x, y : float
the values to be differenced
p : float (optional)
the exponent (default = 2.0)
Returns
-------
result: float
(x - y) ** p
"""
diff = x - y
return diff ** p
Writing myfile.py
# Pydoc is a command-line program bundled with Python
!pydoc -w myfile
wrote myfile.html
from IPython.display import HTML
HTML(open('myfile.html').read())
myfile | index /Users/jakevdp/Opensource/2013_fall_ASTR599/notebooks/myfile.py |
Functions | ||
|
Modules are organized units of code which contain functions, classes, statements, and other definitions.
Any file ending in .py
is treated as a module (e.g. our file myfile.py
above).
Variables in modules have their own scope: using a name in one module will not affect variables of that name in another module.
%%file mymodule.py
# A simple demonstration module
def add_numbers(x, y):
"""add x and y"""
return x + y
def subtract_numbers(x, y):
"""subtract y from x"""
return x - y
Writing mymodule.py
Modules are accessed using import module_name
(with no .py
)
import mymodule
print '1 + 2 =', mymodule.add_numbers(1, 2)
print '5 - 3 =', mymodule.subtract_numbers(5, 3)
1 + 2 = 3 5 - 3 = 2
print add_numbers(1, 2)
--------------------------------------------------------------------------- NameError Traceback (most recent call last) <ipython-input-53-bd09daccae66> in <module>() ----> 1 print add_numbers(1, 2) NameError: name 'add_numbers' is not defined
import mymodule
mymodule.add_numbers(1, 2)
3
from mymodule import add_numbers
add_numbers(1, 2)
3
from mymodule import add_numbers as silly_function
silly_function(1, 2)
3
from mymodule import *
subtract_numbers(5, 3)
2
This final method can be convenient, but should generally be avoided as it can cause name collisions and makes debugging difficult.
Your modules can have their own documentation, can define module-level variables, and can execute code when they load. For example:
%%file mymodule2.py
"""
Example module with some variables and startup code
"""
# this code runs when the module is loaded
print "mymodule2 in the house!"
pi = 3.1415926
favorite_food = "spam, of course"
def multiply(a, b):
return a * b
Writing mymodule2.py
import mymodule2
mymodule2 in the house!
# import again and the initial code does not execute!
import mymodule2
# access module-level documentation
mymodule2?
print mymodule2.multiply(2, 3)
print mymodule2.pi
6
# module variables can be modified
print mymodule2.favorite_food
spam, of course
mymodule2.favorite_food = "eggs. No spam."
print mymodule2.favorite_food
eggs. No spam.
sys
: exposes interactions with the system (environment, file I/O, etc.)os
: exposes platform-specific operations (file statistics, directories, paths, etc.)math
: exposes basic mathematical functions and constantsimport sys
help(sys)
Help on built-in module sys: NAME sys FILE (built-in) MODULE DOCS http://docs.python.org/library/sys DESCRIPTION This module provides access to some objects used or maintained by the interpreter and to functions that interact strongly with the interpreter. Dynamic objects: argv -- command line arguments; argv[0] is the script pathname if known path -- module search path; path[0] is the script directory, else '' modules -- dictionary of loaded modules displayhook -- called to show results in an interactive session excepthook -- called to handle any uncaught exception other than SystemExit To customize printing in an interactive session or to install a custom top-level exception handler, assign other functions to replace these. exitfunc -- if sys.exitfunc exists, this routine is called when Python exits Assigning to sys.exitfunc is deprecated; use the atexit module instead. stdin -- standard input file object; used by raw_input() and input() stdout -- standard output file object; used by the print statement stderr -- standard error object; used for error messages By assigning other file objects (or objects that behave like files) to these, it is possible to redirect all of the interpreter's I/O. last_type -- type of last uncaught exception last_value -- value of last uncaught exception last_traceback -- traceback of last uncaught exception These three are only available in an interactive session after a traceback has been printed. exc_type -- type of exception currently being handled exc_value -- value of exception currently being handled exc_traceback -- traceback of exception currently being handled The function exc_info() should be used instead of these three, because it is thread-safe. Static objects: float_info -- a dict with information about the float inplementation. long_info -- a struct sequence with information about the long implementation. maxint -- the largest supported integer (the smallest is -maxint-1) maxsize -- the largest supported length of containers. maxunicode -- the largest supported character builtin_module_names -- tuple of module names built into this interpreter version -- the version of this interpreter as a string version_info -- version information as a named tuple hexversion -- version information encoded as a single integer copyright -- copyright notice pertaining to this interpreter platform -- platform identifier executable -- absolute path of the executable binary of the Python interpreter prefix -- prefix used to find the Python library exec_prefix -- prefix used to find the machine-specific Python library float_repr_style -- string indicating the style of repr() output for floats __stdin__ -- the original stdin; don't touch! __stdout__ -- the original stdout; don't touch! __stderr__ -- the original stderr; don't touch! __displayhook__ -- the original displayhook; don't touch! __excepthook__ -- the original excepthook; don't touch! Functions: displayhook() -- print an object to the screen, and save it in __builtin__._ excepthook() -- print an exception and its traceback to sys.stderr exc_info() -- return thread-safe information about the current exception exc_clear() -- clear the exception state for the current thread exit() -- exit the interpreter by raising SystemExit getdlopenflags() -- returns flags to be used for dlopen() calls getprofile() -- get the global profiling function getrefcount() -- return the reference count for an object (plus one :-) getrecursionlimit() -- return the max recursion depth for the interpreter getsizeof() -- return the size of an object in bytes gettrace() -- get the global debug tracing function setcheckinterval() -- control how often the interpreter checks for events setdlopenflags() -- set the flags to be used for dlopen() calls setprofile() -- set the global profiling function setrecursionlimit() -- set the max recursion depth for the interpreter settrace() -- set the global debug tracing function FUNCTIONS __displayhook__ = displayhook(...) displayhook(object) -> None Print an object to sys.stdout and also save it in __builtin__._ __excepthook__ = excepthook(...) excepthook(exctype, value, traceback) -> None Handle an exception by displaying it with a traceback on sys.stderr. call_tracing(...) call_tracing(func, args) -> object Call func(*args), while tracing is enabled. The tracing state is saved, and restored afterwards. This is intended to be called from a debugger from a checkpoint, to recursively debug some other code. callstats(...) callstats() -> tuple of integers Return a tuple of function call statistics, if CALL_PROFILE was defined when Python was built. Otherwise, return None. When enabled, this function returns detailed, implementation-specific details about the number of function calls executed. The return value is a 11-tuple where the entries in the tuple are counts of: 0. all function calls 1. calls to PyFunction_Type objects 2. PyFunction calls that do not create an argument tuple 3. PyFunction calls that do not create an argument tuple and bypass PyEval_EvalCodeEx() 4. PyMethod calls 5. PyMethod calls on bound methods 6. PyType calls 7. PyCFunction calls 8. generator calls 9. All other calls 10. Number of stack pops performed by call_function() exc_clear(...) exc_clear() -> None Clear global information on the current exception. Subsequent calls to exc_info() will return (None,None,None) until another exception is raised in the current thread or the execution stack returns to a frame where another exception is being handled. exc_info(...) exc_info() -> (type, value, traceback) Return information about the most recent exception caught by an except clause in the current stack frame or in an older stack frame. exit(...) exit([status]) Exit the interpreter by raising SystemExit(status). If the status is omitted or None, it defaults to zero (i.e., success). If the status is numeric, it will be used as the system exit status. If it is another kind of object, it will be printed and the system exit status will be one (i.e., failure). getcheckinterval(...) getcheckinterval() -> current check interval; see setcheckinterval(). getdefaultencoding(...) getdefaultencoding() -> string Return the current default string encoding used by the Unicode implementation. getdlopenflags(...) getdlopenflags() -> int Return the current value of the flags that are used for dlopen calls. The flag constants are defined in the ctypes and DLFCN modules. getfilesystemencoding(...) getfilesystemencoding() -> string Return the encoding used to convert Unicode filenames in operating system filenames. getprofile(...) getprofile() Return the profiling function set with sys.setprofile. See the profiler chapter in the library manual. getrecursionlimit(...) getrecursionlimit() Return the current value of the recursion limit, the maximum depth of the Python interpreter stack. This limit prevents infinite recursion from causing an overflow of the C stack and crashing Python. getrefcount(...) getrefcount(object) -> integer Return the reference count of object. The count returned is generally one higher than you might expect, because it includes the (temporary) reference as an argument to getrefcount(). getsizeof(...) getsizeof(object, default) -> int Return the size of object in bytes. gettrace(...) gettrace() Return the global debug tracing function set with sys.settrace. See the debugger chapter in the library manual. setcheckinterval(...) setcheckinterval(n) Tell the Python interpreter to check for asynchronous events every n instructions. This also affects how often thread switches occur. setdlopenflags(...) setdlopenflags(n) -> None Set the flags used by the interpreter for dlopen calls, such as when the interpreter loads extension modules. Among other things, this will enable a lazy resolving of symbols when importing a module, if called as sys.setdlopenflags(0). To share symbols across extension modules, call as sys.setdlopenflags(ctypes.RTLD_GLOBAL). Symbolic names for the flag modules can be either found in the ctypes module, or in the DLFCN module. If DLFCN is not available, it can be generated from /usr/include/dlfcn.h using the h2py script. setprofile(...) setprofile(function) Set the profiling function. It will be called on each function call and return. See the profiler chapter in the library manual. setrecursionlimit(...) setrecursionlimit(n) Set the maximum depth of the Python interpreter stack to n. This limit prevents infinite recursion from causing an overflow of the C stack and crashing Python. The highest possible limit is platform- dependent. settrace(...) settrace(function) Set the global debug tracing function. It will be called on each function call. See the debugger chapter in the library manual. DATA __stderr__ = <open file '<stderr>', mode 'w'> __stdin__ = <open file '<stdin>', mode 'r'> __stdout__ = <open file '<stdout>', mode 'w'> api_version = 1013 argv = ['-c', '-f', '/Users/jakevdp/.ipython/profile_default/security/... builtin_module_names = ('__builtin__', '__main__', '_ast', '_codecs', ... byteorder = 'little' copyright = 'Copyright (c) 2001-2013 Python Software Foundati...ematis... displayhook = <IPython.kernel.zmq.displayhook.ZMQShellDisplayHook obje... dont_write_bytecode = False exc_value = TypeError("<module 'sys' (built-in)> is a built-in module"... exec_prefix = '/Users/jakevdp/anaconda' executable = '/Users/jakevdp/anaconda/bin/python' flags = sys.flags(debug=0, py3k_warning=0, division_warn...unicode=0, ... float_info = sys.float_info(max=1.7976931348623157e+308, max_...epsilo... float_repr_style = 'short' hexversion = 34014704 last_value = AttributeError("'module' object has no attribute 'mulitpl... long_info = sys.long_info(bits_per_digit=30, sizeof_digit=4) maxint = 9223372036854775807 maxsize = 9223372036854775807 maxunicode = 65535 meta_path = [] modules = {'ConfigParser': <module 'ConfigParser' from '/Users/jakevdp... path = ['', '/Users/jakevdp/Opensource/Bokeh', '/Users/jakevdp/anacond... path_hooks = [<type 'zipimport.zipimporter'>] path_importer_cache = {'': None, '/Users/jakevdp/.ipython/extensions':... platform = 'darwin' prefix = '/Users/jakevdp/anaconda' ps1 = 'In : ' ps2 = '...: ' ps3 = 'Out: ' py3kwarning = False stderr = <IPython.kernel.zmq.iostream.OutStream object> stdin = <open file '<stdin>', mode 'r'> stdout = <IPython.kernel.zmq.iostream.OutStream object> subversion = ('CPython', '', '') version = '2.7.5 |Anaconda 1.4.0 (x86_64)| (default, Jun 28...3, 22:20... version_info = sys.version_info(major=2, minor=7, micro=5, releaseleve... warnoptions = []
import sys
import os
print "You are using Python version", sys.version
print 40 * '-'
print "Current working directory is:"
print os.getcwd()
print 40 * '-'
print "Files in the current directory:"
for f in os.listdir(os.getcwd()):
print f
You are using Python version 2.7.5 |Anaconda 1.4.0 (x86_64)| (default, Jun 28 2013, 22:20:13) [GCC 4.0.1 (Apple Inc. build 5493)] ---------------------------------------- Current working directory is: /Users/jakevdp/Opensource/2013_fall_ASTR599/notebooks ---------------------------------------- Files in the current directory: .ipynb_checkpoints 00_intro.ipynb 01_basic_training.ipynb 02_advanced_data_structures.ipynb 03_IPython_intro.ipynb 04_Functions_and_modules.ipynb data Healpy.ipynb images Markdown Cells.ipynb myfile.html myfile.py myfile.pyc mymodule.py mymodule.pyc mymodule2.py mymodule2.pyc number_game.py README.txt style.css
Built-in modules are listed at http://docs.python.org/2/py-modindex.html
(source: http://xkcd.com/353/)
# try importing antigravity...
When a script or module is run directly from the command-line (i.e. not imported) a special variable called __name__
is set to "__main__"
.
So, in your module, if you want some part of the code to only run when the script is executed directly, then you can make it look like this:
# all module stuff
# at the bottom, put this:
if __name__ == '__main__':
# do some things
print "I was called from the command-line!"
Here's a longer example of this in action:
%%file modfun.py
"""
Some functions written to demonstrate a bunch of concepts
like modules, import and command-line programming
"""
import os
import sys
def getinfo(path=".",show_version=True):
"""
Purpose: make simple us of os and sys modules
Input: path (default = "."), the directory you want to list
"""
if show_version:
print "-" * 40
print "You are using Python version ",
print sys.version
print "-" * 40
print "Files in the directory " + str(os.path.abspath(path)) + ":"
for f in os.listdir(path):
print " " + f
print "*" * 40
if __name__ == "__main__":
"""
Executed only if run from the command line.
call with
modfun.py <dirname> <dirname> ...
If no dirname is given then list the files in the current path
"""
if len(sys.argv) == 1:
getinfo(".",show_version=True)
else:
for i,dir in enumerate(sys.argv[1:]):
if os.path.isdir(dir):
# if we have a directory then operate on it
# only show the version info
# if it's the first directory
getinfo(dir,show_version=(i==0))
else:
print "Directory: " + str(dir) + " does not exist."
Writing modfun.py
# now execute from the command-line
%run modfun.py
---------------------------------------- You are using Python version 2.7.5 |Anaconda 1.4.0 (x86_64)| (default, Jun 28 2013, 22:20:13) [GCC 4.0.1 (Apple Inc. build 5493)] ---------------------------------------- Files in the directory /Users/jakevdp/Opensource/2013_fall_ASTR599/notebooks: .ipynb_checkpoints 00_intro.ipynb 01_basic_training.ipynb 02_advanced_data_structures.ipynb 03_IPython_intro.ipynb 04_Functions_and_modules.ipynb data Healpy.ipynb images Markdown Cells.ipynb modfun.py myfile.html myfile.py myfile.pyc mymodule.py mymodule.pyc mymodule2.py mymodule2.pyc number_game.py README.txt style.css ****************************************
Note some of the sys
and os
commands used in this script!
This breakout will give you a chance to explore some of the builtin modules offered by Python. For this session, please use your text editor to create the files. You'll have to
Create and edit a new file called age.py
. Though you can do this via the %%file
magic used above, here you should use your text editor.
within age.py
, import the datetime
module
use datetime.datetime()
to create a variable representing your birthday
use datetime.datetime.now()
to create a variable representing the present date
subtract the two (this forms a datetime.timedelta()
object) and print that variable.
Use this object to answer these questions:
How many days have you been alive?
How many hours have you been alive?
What will be the date 1000 days from now?
Create and edit a new file called age1.py
. When run from the command-line with one argument, age1.py
should print out the date in that many days from now. If run with three arguments, print the time in days since that date.
[~]$ python age1.py 1000
date in 1000 days 2016-06-06 14:46:09.548831
[~]$ python age1.py 1981 6 12
days since then: 11779