Difference between revisions of "Python-Perl cheatsheet"

Revision as of 12:13, 8 May 2011

Cheatsheet

This is so far based on my study of Learning Perl (O'Reilly), also known as "the llama book". See also the Perl Phrasebook on wiki.python.org

what	Python	Perl	notes
null type	None	undef	not exactly identical
String concatenation	"py"+"thon"	"pe"."rl"
	if x == y	if ( $x == $y ) and if ( $x eq $y)	Perl requires parenthesis in a comparison (same for while). Perl uses different comparators for numbers and strings
Arrays
declare a list	[1,2,3]	(1,2,3)	note that arrays contain only scalars! (1,2, (3,4), 5) becomes (1,2,3,4,5)
array length	len(array)	$#array + 1	Python returns the length of array. Perl gives me the index of the last element. Also, the array value in a scalar context is (usually) its size.
	range(1,100)	(1..100)
create a list from whitespaced values	"fred barney wilma dino".split()	qw( fred barney wilma dino);	You can use separators others than parenthesis, e.g. qw ! fred barney wilma dino !
multiple assignment	fred, barney, dino = "flintstone", "rubble", None	($fred, $barney, $dino) = ("flintstone", "rubble", undef);
array concatenation	a+b	(@a, @b)	you can also mix like (@a, 3, @b, undef)
	array.append(item)	push @array, item;	See also Perl's unshift() for attaching to the front of array
	val = array.pop()	val = pop @array;	See also Perl's shift() for removing from the front of array
List iteration	for item in array	foreach $item (@array)	Note that $item is the real element of @array, so if you modify $item, you modify the array content
	list.reverse()	@list = reverse @list;	Python is in place, Perl is not. Same for sort.
Mappings (dictionaries / hashes)
Define hash	dict = { 'a':1 , 'b':2 }	hash = ( 'a'=> 1, 'b'=> 2, )
Retrieve value	dict[key]	$hash{$key}
Invert a hash	inverted_dict = dict([[v,k] for k,v in mydict.items()])	%inverted_hash = reverse %hash;
Keys	keys = dict.keys()	@keys = keys %hash;
Values	vals = dict.values()	@vals = values %hash;
Iterating a hash	for key in dict.keys(): dict[key]...	while ( ($key, $val) = each %hash) {...}	Notice the each function -iterates and the assigment is false when there are no more couples. Notice also, if you want keys sorted, you can use foreach $key (sort keys %hash) {...} , which is similar to the Python case.
Functions and subroutines
Subroutine/function declaration	def function(): ...	sub function { ... }
	return x	return x or [last evaluated expression is return value]	Perl has an explicit return statement, but it's facultative. A return-less sub won't return "undef", but the last expression evaluated!
Arguments of a function	def function(arg1,arg2...)	sub function { ... $_[0] , $[1] ... }	Perl doesn't have explicit arguments; args are stored in the @_ default array.
Name of argument variables	def function(arg1,arg2...)	sub function { my($arg1, $arg2) = @_; }	You can name arguments by declaring them using the @_ default array
Static variables in function scope	(define a generator ; see yield ) ; (use namespace within function, like def foo(): foo.myvar=1)	state $myvar=1;	Defines a variable which is statically defined but within function namespace -so it's not resetted when function is called, it maintains its state.
Input / output
	x = raw_input()	$x = <STDIN>;	Interestingly if you use a list context, like in @x = <STDIN> , you get multiline stdin input.
	sys.argv	@ARGV
Program's own name	sys.argv[0]	$0
Multiple file arguments stream	see fileinput* module*	<>	Comntains sequentially all the file stream from all files in program arguments, like ./script.pl file1 file2 file3
Open a file stream for reading	f = open('test.txt')	open TEST, "test.txt";	Use uppercase, it's good practice.
Open a file stream for writing	f = open('test.txt','w')	open TEST, ">test.txt";
Open a file stream for appending	f = open('test.txt','a')	open TEST, ">>test.txt";
Close file	f.close()	close TEST;
Error handling
Raise exception	raise	die
Operating system interaction
Environment variables	os.environ	%ENV

Traps

Static arrays

To have a valid

state @array

you need to

use 5.010;

while/foreach memory consumption

This:

while (<STDIN>) { ...

reads one line at a time.

This:

foreach (<STDIN>) { ...

instead loads all STDIN into memory before processing it!

Print a hash

If you do

print "%hash";

it will NOT interpolate and write the hash! it will dumbly write the string "%hash".

You can unwind the array by @unwind_hash = %hash (but it is difficult to read) or you have to write your helper function.

@@ Line 221: / Line 221: @@
 [[Category:Programming]]
+===Print a hash===
+*If you do
+:<code>print "%hash";</code>
+:it will NOT interpolate and write the hash! it will dumbly write the string "%hash".
+:You can unwind the array by <code>@unwind_hash = %hash</code> (but it is difficult to read) or you have to write your helper function.

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