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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.

Learning Python

Related Reading

Learning Python
By Mark Lutz, David Ascher

Operator Description A
`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, x>>y Left-shift, right-shift L
x&y Bitwise AND L
x^y Bitwise XOR L
x|y Bitwise OR L
x<y, x<=y, x>y, 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
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
output = open('/tmp/spam', 'w') Create output file ('w' means write).
input = open('data', 'r') Create input file ('r' means read).
S = ) Read entire file into a single string.
S = 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
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
( ) 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

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