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--- notes:python_cheat_sheet [2026/09/14 13:36]
169.224.104.252 old revision restored (2026/08/06 06:28)
+++ notes:python_cheat_sheet [2026/09/16 07:35] (current)
85.106.99.254 old revision restored (2026/07/20 14:51)
@@ Line 40: / Line 40: @@
   * Lists can be nested.
   * 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]
-[[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)).
@@ Line 54: / Line 55: @@
   * 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.
-  * format-string % (tuple) allows use of printf formatting strings. Also string.format() .
 ===== Control Flow =====
@@ Line 85: / Line 85: @@
   * zip takes two or more lists with the same number of elements and creates a single list of lists with corresponding members grouped together.
-===== Modules =====
+===== Modules & Packages =====
   * A //module// is a file of python definitions.
-  * Definitions in a module can be accessed by importing the module. <code python> import fib
+  * Definitions in a module can be accessed by importing the module. <code python>import fib
 from fib import fib1, fib2
 </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.
   * When a module is imported, a byte-code version is stored to module.pyc which speeds up successive loads.
- * There are many standard modules including sys which is common to all platforms.
+  * There are many standard modules including sys which is common to all platforms.
+  * dir(module) lists all names in a module sorted alphabetically. dir(builtins) shows all the built-in functions and variables.
+  * 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.
+===== Input/Output =====
+  * str() returns a readable representations, repr() returns a canonical representation
+  * Strings have several formatting methods.
+  * //formatstring.format(tuple)// is the [[http://docs.python.org/3.0/library/string.html#formatspec | new-style printf]] equivalent. The [[http://docs.python.org/3.0/library/stdtypes.html#old-string-formatting | old-style]] was //formatstring % (tuple)// which uses C-type format strings.
+  * To open a file <code python> open('/tmp/workfile', 'w') </code>
+  * [[http://docs.python.org/3.0/library/stdtypes.html#file-objects | File methods]] include read, readline, write.
+  * //pickle// contains [[http://docs.python.org/3.0/library/pickle.html | load and dump methods]] for serializing objects. Support should be added for new object types.

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