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notes:python_cheat_sheet [2009/04/07 19:59]
smthng 		notes:python_cheat_sheet [2009/12/18 18:06]
smthng
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 ====== Python Cheat Sheet ====== ====== Python Cheat Sheet ======
  
-===== Overview ===== +[[http://www.python.org/ | Python]] is a dynamic object-oriented programming language with extensive standard libraries. These notes summarize the [[http://docs.python.org/3.1/tutorial/ | Python 3.1 tutorial]]. 
- +  * **python**/**python3.1** is the command-line interpreter with readline where available.
-[[http://www.python.org/ | Python]] is a dynamic object-oriented programming language with extensive standard libraries. These notes summarize the [[http://docs.python.org/3.0/tutorial/ | Python 3.0 tutorial]]. +
- +
-===== The Interpreter ===== +
- +
-  * **python**/**python3.0** is the command-line interpreter with readline where available.+
   * python -c command runs a single command.   * python -c command runs a single command.
-  * sys.argv is a list of strings with the script if any in sys.argv[0] +  * %%>>> prompts for a line . ... prompts for a continuation of a previous line.%%
-  * >>> prompts for a line . ... prompts for a continuation of a previous line+
   * PYTHONSTARTUP can point to a file of startup commands   * PYTHONSTARTUP can point to a file of startup commands
   * Expressions are evaluated and printed.   * Expressions are evaluated and printed.
   * The last printed expression is assigned to _ .   * The last printed expression is assigned to _ .
- 
  
 ===== Basics ===== ===== Basics =====
   * Comments start with # .   * Comments start with # .
 +  * sys.argv is a list of strings with the script if any in sys.argv[0]
   * Variables can be assigned (using = ) without being defined but must be assigned before being used.   * Variables can be assigned (using = ) without being defined but must be assigned before being used.
   * a,b = c,d does a multiple assignment.   * a,b = c,d does a multiple assignment.
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   * lambda x: x + n creates a small anonymous function.   * lambda x: x + n creates a small anonymous function.
   * A function should contain a triply quoted doc string at the start. This is accessible with func.__doc__ .   * A function should contain a triply quoted doc string at the start. This is accessible with func.__doc__ .
-  * 4 spaces per indent, no tabs. CamelCasel for classes lower_case_with_underscores for functions.+  * 4 spaces per indent, no tabs. CamelCase for classes lower_case with underscores for functions.
  
 ===== Lists ===== ===== Lists =====
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   * Lists can be nested.   * Lists can be nested.
   * Other methods include append, pop, extend, index, count, sort, reverse.   * Other methods include append, pop, extend, index, count, sort, reverse.
-  * Lists can be created from sequences e.g <code python>+  * Lists can be created from sequences using list comprehension <code python>
 vec = [2, 4, 6] vec = [2, 4, 6]
-[[x, x**2] for x in vec] +[[x, x**2] for x in vec if x > 2] 
-[[2, 4], [4, 16], [6, 36]]</code>+[[4, 16], [6, 36]] 
 +[x+y for x in vec for y in vec2 if x > 2]</code>
   * Other sequence types include tuples ( (1,2) or 1,2) and sets ({1,2} or set(1,2)).   * Other sequence types include tuples ( (1,2) or 1,2) and sets ({1,2} or set(1,2)).
  
-===== Strings =====+===== Input/Output =====
   * Strings can be enclosed with double or single-quotes.   * Strings can be enclosed with double or single-quotes.
   * \ in a string by itself goes to the next line but does not insert a newline (\n does).   * \ in a string by itself goes to the next line but does not insert a newline (\n does).
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   * A string can be treated as a list of letters.   * A string can be treated as a list of letters.
   * But strings can't be modified using slices or indexes.   * But strings can't be modified using slices or indexes.
-  * [[http://docs.python.org/3.0/library/stdtypes.html#string-methods | String methods]] include strip, format, capitalize etc. +  * [[http://docs.python.org/3.1/library/stdtypes.html#string-methods | String methods]] include strip, format, capitalize etc. 
-  * format-string % (tuple) allows use of printf formatting strings. Also string.format() .+  * //formatstring.format(tuple)// is the [[http://docs.python.org/3.1/library/string.html#formatspec | new-style printf]] equivalent. The [[http://docs.python.org/3.1/library/stdtypes.html#old-string-formatting | old-style]] was //formatstring % (tuple)// which uses C-type format strings. 
 +  * str() returns a readable representation, repr() returns a canonical representation of any object. 
 +  * To open a file <code python> open('/tmp/workfile', 'w') </code> 
 +  * [[http://docs.python.org/3.1/library/stdtypes.html#file-objects | File methods]] include read, readline, write. 
 +  * //pickle// contains [[http://docs.python.org/3.1/library/pickle.html | load and dump methods]] for serializing objects. Support should be added for new object types. 
 + 
  
 ===== Control Flow ===== ===== Control Flow =====
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   * Dictionaries are unordered sets of key value pairs e.g. { a:1, b:2} or dict([(a,1),(b,2)]) or dict(a=1,b=2)   * Dictionaries are unordered sets of key value pairs e.g. { a:1, b:2} or dict([(a,1),(b,2)]) or dict(a=1,b=2)
   * Dictionary keys/values/items can be enumerated over.   * Dictionary keys/values/items can be enumerated over.
-  * sorted and reversed take in liats and return new lists.+  * sorted and reversed take in lists and return new lists.
   * enumerate takes in a list and returns a list of pairs where the first item in the pair is an integer starting with 0.   * enumerate takes in a list and returns a list of pairs where the first item in the pair is an integer starting with 0.
   * zip takes two or more lists with the same number of elements and creates a single list of lists with corresponding members grouped together.   * zip takes two or more lists with the same number of elements and creates a single list of lists with corresponding members grouped together.
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 from fib import fib1, fib2 from fib import fib1, fib2
 </code> </code>
-  * __name__ is set to the name of the module or to __main__ if called from the command line.+  * %%__name__%% is set to the name of the module or to %%__main__%% if called from the command line.
   * The search list for modules is the variable sys.path initialized from the directory containing the input script (or the current directory), PYTHONPATH and the installation-dependent default.   * The search list for modules is the variable sys.path initialized from the directory containing the input script (or the current directory), PYTHONPATH and the installation-dependent default.
   * When a module is imported, a byte-code version is stored to module.pyc which speeds up successive loads.   * When a module is imported, a byte-code version is stored to module.pyc which speeds up successive loads.
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   * Packages can be used to group modules. Packages should be in a named directory with an __init__.py file optionally containing initialization code. E.g. import sound.effects.echo .   * Packages can be used to group modules. Packages should be in a named directory with an __init__.py file optionally containing initialization code. E.g. import sound.effects.echo .
   * . is the current package, .. is the parent package etc.   * . is the current package, .. is the parent package etc.
 +
 +===== Error Handling =====
 +
 +  * Syntax errors are parsing errors, exceptions are runtime errors.
 +  * Exceptions are class objects in the exceptions module. [[http://docs.python.org/3.1/library/exceptions.html#bltin-exceptions | List of builtin exceptions]].
 +  * Exception handling is supported using try..except..else..finally :<code python>
 +try:
 +    result = x / y
 +except ZeroDivisionError:
 +    print("division by zero!")
 +except Exception as inst:
 +    print(type(inst))    # the exception instance
 +    print(inst.args)     # arguments stored in .args
 +    print(inst)          # also prints .args
 +else:
 +    print("result is", result)
 +finally:
 +    print("executing finally clause")
 +</code>
 +  * Errors can be raised using ''raise Exception('mine')'' or just ''raise'' in an exception handler.
 +  * New exception classes can be created and should derive from the Exception class.
 +  * ''with'' can be used to automatically call the cleanup actions of an object e.g. <code python>
 +with open("myfile.txt") as f:
 +    for line in f:
 +        print(line)
 +</code>
 +
 
 
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