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--- notes:python_cheat_sheet [2026/09/14 01:55]
216.73.217.14 old revision restored (2026/09/13 22:52)
+++ notes:python_cheat_sheet [2026/09/16 11:25] (current)
216.73.216.94 old revision restored (2026/09/13 08:35)
@@ Line 22: / Line 22: @@
   * Imaginary numbers can be created using complex(a,b) or a+bJ. .real and .imag will extract real and imaginary parts.
   * print("a","b",end="") prints a b with no newline at the end.
+===== Functions =====
+  * To define a function or procedure <code python>define func(n,str="default"):</code> <code python>define func(formal1, formal2, *arguments **keywords)</code>
+  * The default value for an argument is evaluated only once when the function is defined.
+  * All arguments are passed using //call by object reference//. So changes are seen by caller.
+  * * unpacks a list or dictionary for use in a 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__ .
+  * 4 spaces per indent, no tabs. CamelCasel for classes lower_case_with_underscores for functions.
 ===== Lists =====
@@ Line 27: / Line 36: @@
   * list[4] is 4th element. list[2:4] is second and third elements. list[:2] is first 2 elements. list[2:] is third element to last one. list[:] is the whole list.
   * Negative indexes count from last element.
-  * List slices can be modified. list[3:3] can be used to insert elements at position 3. list.append() appends to the list.
+  * List slices can be modified. list[3:3] can be used to insert elements at position 3.
   * len(list) returns length of the list.
   * Lists can be nested.
+  * Other methods include append, pop, extend, index, count, sort, reverse.
+  * Lists can be created from sequences using list comprehension <code python>
+vec = [2, 4, 6]
+[[x, x**2] for x in vec if x > 2]
+[[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)).
 ===== Strings =====
@@ Line 39: / 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 =====
+  * Controlled by level of indentation.
+  * //pass// is a NOOP .
+  * <code python>
+while a<10:
+  pass</code>
+  * <code python>
+for x in list:
+  pass
+for x in range(1,5):
+  pass</code>
+  * <code python>
+if a›10:
+   pass
+elif a›5:
+   pass
+else:</code>
+  * //break// and //continue// can be used
+  * Loops may have else clauses for code executed after normal termination (not a break).
+===== Assorted Collection Stuff =====
+  * 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.
+  * sorted and reversed take in liats 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.
+  * 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 & Packages =====
+  * A //module// is a file of python definitions.
+  * 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.
+  * 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.
+  * 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.
+  * [[ http://docs.python.org/3.0/library/string.html#formatspec | formatstring.format(tuple)]] is the printf equivalent. The old style was [[http://docs.python.org/3.0/library/stdtypes.html#old-string-formatting | formatstring % (tuple)]] which uses C-type format strings.
+  * To open a file <code python> open('/tmp/workfile', 'w') </code>
+  * File methods include read, readline, write.
+  * //pickle// contains load and dump methods for serializing objects. Support should be added for new object types.

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