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O'Reilly Book Excerpts: Python In a Nutshell; Learning Python, 2nd edition

## A Python Quick Reference to Useful Commands

Editor's note: O'Reilly's Python books have been a staple source of quality information for programmers for years. If you haven't visited our Python Resource Center in a while, it may be time to stop by again. There you'll find two excellent resources: Learning Python, 3rd Edition and Python in a Nutshell, 2nd Edition. The former offers Python programmers a comprehensive learning tool for Python and object-oriented programming; the latter offers programmers one-stop shopping when they need help remembering or deciphering Python's syntax and its many modules.

In this excerpt, plucked from the pages of these two books, you'll find a quick reference to many useful Python commands, covering object methods, common file operations, and much more. And we've made it available for download as a PDF (55K) for you to print and post where you program. Enjoy.

Operator Description A OPERATOR PRECEDENCE IN EXPRESSIONS `expr,...` String conversion NA {key:expr,...} Dictionary creation NA [expr,...] List creation NA (expr,...) Tuple creation or simple parentheses NA f(expr,...) Function call L x[index:index] Slicing L x[index] Indexing L x.attr Attribute reference L x**y Exponentiation (x to yth power) R ~x Bitwise NOT NA +x, -x Unary plus and minus NA x*y, x/y, x//y, x%y Multiplication, division, remainder L x+y, x-y Addition, subtraction L x<>y Left-shift, right-shift L x&y Bitwise AND L x^y Bitwise XOR L x|y Bitwise OR L xy, x>=y Comparisons C x<>y, x!=y, x==y Equality/inequality tests* C x is y, x is not y Identity tests C x in y, x not in y Membership tests C not x Boolean NOT NA x and y Boolean AND L x or y Boolean OR L lambda arg,...: expr Anonymous simple function NA * x!=y and x<>y are the same inequality test (!= is the preferred form, <> obsolete) A – Associativity L – Left R – Right C – Chaining NA – Not associative

Operator Description LIST OBJECT METHODS L.count(x) Returns the number of occurrences of x in L L.index(x) Returns the index of the first occurrence of x in L or raises an exception if L has no such item L.append(x) Appends x to the end of L L.extend(l) Appends all the items of list l to the end of L L.insert(i,x) Inserts x at index i in L L.remove(x) Removes the first occurrence of x from L L.pop(i=-1) Returns the value of the item at index i and removes it from L L.reverse( ) Reverses, in-place, the items of L L.sort(f=cmp) Sorts, in-place, the items of L, comparing items by f

Operation Interpretation COMMON FILE OPERATIONS output = open('/tmp/spam', 'w') Create output file ('w' means write). input = open('data', 'r') Create input file ('r' means read). S = input.read( ) Read entire file into a single string. S = input.read(N) Read N bytes (1 or more). S = input.readline( ) Read next line (through end-line marker). L = input.readlines( ) Read entire file into list of line strings. output.write(S) Write string S into file. output.writelines(L) Write all line strings in list L into file. output.close( ) Manual close (done for you when file collected).

Operation Interpretation COMMON DICTIONARY LITERALS AND OPERATIONS D1 = { } Empty dictionary D2 = {'spam': 2, 'eggs': 3} Two-item dictionary D2['eggs'] Indexing by key D2.has_key('eggs'), 'eggs' in D2 membership test D2.keys( ), D2.values( ), D2.items( ) lists of keys, values, items D2.copy( ), D2.update(D1) shallow copy, dict merging D2.get(key, default=None) "indexing" w/default value len(D1) Length (number stored entries) D2[key] = 42 Adding/changing del D2[key] deleting D4 = dict(zip(keyslist, valslist)) Construction

Operation Interpretation COMMON TUPLE LITERALS AND OPERATIONS ( ) An empty tuple T1 = (0,) A one-item tuple (not an expression) T2 = (0, 'Ni', 1.2, 3) A four-item tuple T2 = 0, 'Ni', 1.2, 3 Another four-item tuple (same as prior line) T1[i] Indexing T1[i:j] slicing len(t1) length (number of items) T1 + T2 Concatenation T2 * 3 repetition for x in T2 Iteration 3 in T2 membership