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--- notes:python_cheat_sheet [2026/09/13 22:52]
43.166.174.45 old revision restored (2009/04/02 17:40)
+++ notes:python_cheat_sheet [2026/09/14 00:17] (current)
14.171.170.69 old revision restored (2026/07/27 06:07)
@@ Line 3: / Line 3: @@
 ===== 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/3.0/tutorial/ | Python 3.0 tutorial]].
+[[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.
-===== The Interpreter =====
-  * **python**/**python3.0** is the command-line interpreter with readline where available.
   * 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
   * Expressions are evaluated and printed.
   * The last printed expression is assigned to _ .
 ===== Basics =====
   * 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.
   * a,b = c,d does a multiple assignment.
   * 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. CamelCase for classes lower_case with underscores for functions.
 ===== Lists =====
@@ Line 27: / Line 32: @@
   * 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 38: / Line 50: @@
   * A string can be treated as a list of letters.
   * 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() .
 ===== 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 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.
+  * 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.
+  * //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.
+  * 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.
+===== 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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