Python-Perl cheatsheet


This is so far based on my study of Learning Perl (O'Reilly), also known as "the llama book", plus stuff I found on the web.

See also the Perl Phrasebook on

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
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);
pretty-print array print array print "@array\n"; Simply using print @array will print the contents but without spacing, so that (1,2,3) becomes 123.
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 $item, you modify the array content
list.reverse() @list = reverse @list; Python is in place, Perl is not. Same for sort.
Check if item in list if item in list :... if ( grep {$_ eq $element} @array ) { ... TIMTOWDI; see also using hashes and exists()
Index of item list.index(item) my( $index )= grep { $array[$_] eq $search_for } 0..$#array; See
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.
Creating an hash from two arrays for key,val in zip(keys,vals): dict[key]=val @dict{@keys} = @vals See hash slicing.
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.
Returning more than one array/dictionary return a,b return \@array, \%hash; In Perl, functions collapse all input in a single array and all output in a single array. You have to play with references.
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 ./ 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
Catch exception try: ... except: ... eval { ... } or do {...}
Operating system interaction
Environment variables os.environ %ENV
Save data structures to disk pickle module Storable module Perl has a quite long list of modules to save data structures to disk


Static arrays

To have a valid
state @array
you need to
use 5.010;

while/foreach memory consumption

while (<STDIN>) { ...
reads one line at a time.
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.