Part IV. More on Python. Tobias Neckel: Scripting with Bash and Python Compact Max-Planck, February 16-26,

Transcription

1 Part IV More on Python Compact Max-Planck, February 16-26,

2 More on Strings Special string methods (excerpt) s = " Frodo and Sam and Bilbo " s. islower () s. isupper () s. startswith (" Frodo ") # try s. startswith (" Frodo ", 2) s. endswith (" Bilbo ") s = s. strip () s. upper () s = s. lower () s. capitalize () # capitalize first character s = s. center ( len (s )+4) # center ( padding default : space ) s. lstrip () s. rstrip (" ") s = s. strip () s. find (" sam ") s. rfind (" and ") Compact Max-Planck, February 16-26,

3 More on Strings (2) Searching, splitting and joining (excerpt) s. find (" sam ") s. rfind (" and ") s. replace (" and ", "or") s. split () # or s. split ( None ); arbitrary numb. whitesp. s. split (" ") s. split (" and ") s. split (" and ", 1) s = """ Line by Line """ s. splitlines () ", ". join ([" sequence ", "of", " strings "]) Compact Max-Planck, February 16-26,

4 More on Built-in Sequences Dictionaries Map keys to values, arbitrary types Only immutable objects as keys Can serve as database d = {} d[" Bilbo "] = " Hobbit " d[" Elrond "] = " Elf " d [42] = 1234 d. has_key (" Bilbo ") d. keys () # unordered d. items () d. values () del d [42] d. clear () Compact Max-Planck, February 16-26,

5 More on Functions Functions have to be defined before actually called for the first time Note: Python is an interpreted language Variables Variables defined or assigned to in functions have local scope Global variables (surrounding context) can be accessed To modify global variables use global count = 0... def count_it (n): global count count += 1 In general: avoid global variables (pass as params instead) Compact Max-Planck, February 16-26,

6 Call by Reference vs. Call by value Call by reference: address (id) of parameter is handed over Call by value: value of parameter is handed over In Python: always call by reference But: assignment changes reference within function >>> def add_one (x): >>> x = x + 1 >>> >>> x = 3 >>> add_one (x) >>> x 3 Compact Max-Planck, February 16-26,

7 Call by Reference vs. Call by value (2) After assignment: local variable references to a new object. Using methods, the original object can be modified. >>> def append1 (l): >>> l = l + [4] >>> >>> def append2 (l): >>> l. append (4) >>> >>> l = [1,2,3] >>> append1 (l); l [1,2,3] >>> append2 (l); l [1,2,3,4] Compact Max-Planck, February 16-26,

8 More on Functions (2) Return values Functions can have multiple return values (returned as tuple) def get_hobbits (): return " Bilbo ", " Frodo " h = get_hobbits () (a, b) = get_hobbits () Assignment rules correspond to slice assignment l = [4, 2] (a, b) = l # implicit type conversion a, b = l l [0:3] = (3, 2) # implicit type conversion Compact Max-Planck, February 16-26,

9 More on Functions (3) Docstrings Functions (and classes, modules,...) can provide help text String after function header (can be multiline) def answer (): " Returns an answer to a question " print " answer " help ( answer ) Compact Max-Planck, February 16-26,

10 Arithmetic Operations Classical operations x + y # Addition x - y # Subtraction x * y # Multiplication x / y # Division x // y # Truncated division x ** y # Exponentiation x % y # Modulo x -= 2; x *= 4;... Division differs for int and float (Python < 3.0) 7/4 7.0/4 Compact Max-Planck, February 16-26,

11 More Operations General functions abs (x) divmod (x, y) # (x // y, x % y) pow (x, y [, modulo ]) # x**y % modulo round (x, [n]) # round to 10**( - n) Operations on integers x << y # Left shift x >> y # Right shift x & y # Bitwise and x y # Bitwise or x ^ y # Bitwise xor ~ x # Bitwise negation Compact Max-Planck, February 16-26,

12 File I/O Opening files Create file object (text files) fd = open (" testfile. txt ") # read fd = open (" testfile. txt ", r ) # read fd = open (" testfile. txt ", w ) # write fd = open (" testfile. txt ", a ) # append Create file object (binary files) fd = open (" testfile. txt ", rb ) # read fd = open (" testfile. txt ", wb ) # write fd = open (" testfile. txt ", ab ) # append open has more options (encoding, how to handle newlines,... ) Compact Max-Planck, February 16-26,

13 File I/O (2) Methods of file objects Reading fd. read () # all fd. read (n) # n Bytes fd. readline () # one line fd. readlines () # Writing fd. write (" New Text \n") fd. writelines ([" first line \n", " Second \n"]) Don t forget to write newlines Closing fd. close () Compact Max-Planck, February 16-26,

14 File I/O (3) Iterating over textfile for loop over file line by line fd = open (" fulltext. txt ") for line in fd: # rather than : in fd. readlines () print line # or: while True : line = fd. readline () if not line : break print line Compact Max-Planck, February 16-26,

15 File I/O (3) Iterating over textfile for loop over file line by line fd = open (" fulltext. txt ") for line in fd: # rather than : in fd. readlines () print line # or: while True : line = fd. readline () if not line : break print line Some more functions on file objects fd.tell() get current file position fd.seek(offset) set file to position fd.flush() flush output buffer. Note: buffered for efficiency Compact Max-Planck, February 16-26,

16 Getting more... Modules Outsource functionality in separate.py files Import them as library module Example (tools.py): """ This module provides some helper tools. Try it. """ counter = 42 def readfile ( fname ): " Read text file. Returns list of lines " fd = open ( fname, r ) data = fd. readlines () fd. close () return data def do_nothing (): "Do really nothing " pass Compact Max-Planck, February 16-26,

17 Modules (2) Import module import tools tools. do_nothing () print tools. counter Import module and change name import tools as t t. do_nothing () Import selected symbols to current namespace from tools import do_nothing, readfile from tools import counter as cntr do_nothing () print cntr Compact Max-Planck, February 16-26,

18 Modules (3) Import all symbols to current namespace from tools import * do_nothing () print counter Modules can control which symbols are imported by from module import *: # module tools. py all = [ readfile, counter ] Then do_nothing() is unknown after import * Compact Max-Planck, February 16-26,

19 Modules (3) Import all symbols to current namespace from tools import * do_nothing () print counter Modules can control which symbols are imported by from module import *: # module tools. py all = [ readfile, counter ] Then do_nothing() is unknown after import * Inspect namespace Inspect namespace of module with dir ( tools ) Compact Max-Planck, February 16-26,

20 Modules (4) Getting help Access docstrings import tools help ( tools ) help ( tools. do_nothing ) Compact Max-Planck, February 16-26,

21 Modules (4) Getting help Access docstrings import tools help ( tools ) help ( tools. do_nothing ) Execute module as main program tools.py should serve as program and module # tools.py... if name == main : print " tools.py executed " else : print " tools.py imported as module " Compact Max-Planck, February 16-26,

22 Modules (5) Reload module When debugging a module reload with reload (Python<3.0) reload ( tools ) Module search path Modules have to be in current directory or in directory in search path import sys sys. path. append (" folder /to/ module ") import... Automatically extend sys.path by setting environment variable PYTHONPATH Compact Max-Planck, February 16-26,

23 Packages Group modules Modules can be grouped together Folder structure determines modules, e.g.: tools / init. py # contents for " import tools " files. py # for " import tools. files " graphics. py # for " import tools. graphics " stringtools / init. py # for " import tools. stringtools "... further nesting If from tools import * should import submodules, tools/ init.py has to contain all = [" files ", " graphics "] Compact Max-Planck, February 16-26,

24 Command-Line Options Using sys sys.argv is list of command-line options First one (sys.argv[0]) is program name import sys print " Executing : %s" % ( sys. argv [0]) if len ( sys. argv ) < 2: print " Not enough parameters " sys. exit (1) print " Parameters :" print ", ". join ( sys. argv [1:]) Parse parameters... Compact Max-Planck, February 16-26,

25 Command-Line Options (2) Module argparse Class ArgumentParser in module argparse simplifies option parsing #!/ usr / bin / python import argparse p = argparse. ArgumentParser () # specify options p. add_argument ("-o", action =" store ", dest =" opt ") p. add_argument ("-v"," -- verbose ",action =" store_true ", dest =" verbose ", help =" Produce more output ") # parse options options = p. parse_args () if options. verbose : print " Starting program... " print options Compact Max-Planck, February 16-26,

26 Module argparse (2) Program now supports Default help Two custom parameters Try (parameters.py):./ parameters.py./ parameters. py -h./ parameters.py -o " Enjoy it" -- verbose./ parameters.py -v file1. txt file2. txt Compact Max-Planck, February 16-26,

27 Module argparse (3) Some parameters of add_argument One character "-x" and/or multiple character switches "--xyz" action can be store, store_true, store_false, append, count dest name of option default default value help help text type one of string (default), int, long, float, complex, choice choices = [ first, second, third ] for type= choice Compact Max-Planck, February 16-26,

28 Module argparse (3) Some parameters of add_argument One character "-x" and/or multiple character switches "--xyz" action can be store, store_true, store_false, append, count dest name of option default default value help help text type one of string (default), int, long, float, complex, choice choices = [ first, second, third ] for type= choice Help text with set_usage Reference to program name with %prog p = argparse. ArgumentParser ( % prog [ options ] file (s) Do nice things with file (s) ) Compact Max-Planck, February 16-26,

29 Standard Input, Output, and Error Module sys provides three standard file objects sys.stdin read only sys.stdout, sys.stderr write only import sys sys. stdout. write (" Enter line :\n") # = print "..." line = sys. stdin. readline () # or: line = raw_input (" Enter line :\ n") if error : sys. stderr. write (" Error!\n") write does not add newlines raw_input([text]) strips endling newline Input and output buffered Compact Max-Planck, February 16-26,

30 More on Lists List comprehension Short notation to create lists even_squares = [] for i in range (10): if i%2 == 0: even_squares. append (i*i) even_squares = [ i **2 for i in range (10) if i %2==0] Compare: { i 2 i {0,..., 9}, i even } Compact Max-Planck, February 16-26,

31 More on Lists List comprehension Short notation to create lists even_squares = [] for i in range (10): if i%2 == 0: even_squares. append (i*i) even_squares = [ i **2 for i in range (10) if i %2==0] Compare: { i 2 i {0,..., 9}, i even } Can contain more than one for... in... [if...] [(x,y. upper ()) for x in range (4) if x%2 == 0 for y in ["a", "b", "c"]] Compact Max-Planck, February 16-26,

32 More on Lists (2) Advanced slice assignement Note: assigning slices of mutable sequences can change size l = range (5) l [1:3] = [7,8,9] l [4:6] = [6] Compact Max-Planck, February 16-26,

33 More on Lists (2) Advanced slice assignement Note: assigning slices of mutable sequences can change size l = range (5) l [1:3] = [7,8,9] l [4:6] = [6] range and xrange range([i,] j [, stride]) creates list Memory allocated for whole list, even when only iterating over them, especially in loops xrange object calculates values when accessed (generator) for i in range (650000): do_something () for i in xrange (650000): do_something () Compact Max-Planck, February 16-26,

34 Even more on Built-in Sequences set and frozenset set is mutable, frozenset is not s = set ([3,1,2,3,"foo ",2]) len (s) l = [2,8,7] # any iterable sequence s. difference (l) s. intersection (l) s. union (l) s. symmetric_difference (l) s. issubset ([3,"foo "]) s. issuperset ([3,"foo "]) For set s. add (42) s. remove (3) s. intersection_update (l) # and more methods... Compact Max-Planck, February 16-26,

35 Even more on Functions Anonymous functions Can be defined using keyword lambda Lambda functions allow functional programming def inc (i): return i+1 inc = lambda (i): i+1 ( lambda i: i +1)(4) Common use: map function on list items / filter list l = range (10) map ( lambda x: x*x+1, l) filter ( lambda x: x %2==0, l) Compact Max-Planck, February 16-26,