Essential Python 3 code for lists

# Create a list mylist = [] # Add an element to the front or back mylist.append( "abc" ) mylist.insert( 0, "def" ) # Pop element off front or back end = mylist.pop() start = mylist.pop( 0 ) # Forward iterate over elements for item in mylist: print( item ) # Get the length of list len( mylist ) # Test if empty if not mylist: print( "list is empty" ) 

# Get item at location mylist[2] # Insert an item at location mylist.insert( 3, "abc" ) # Remove an item from location del mylist[2] # Replace/Assign item at location mylist[1] = "def" 

# Find an item if item in mylist: index = mylist.index(item) # Using `index` and error handling try: index = mylist.index( 'abc' ) except ValueError: index = None # Using `next` and filtering next((x for x in mylist if x == 'ghif'), None) # Find and remove an item if item in mylist: mylist.remove( item ) # with error handling try: mylist.remove( item ) except ValueError: pass # Find last matching item # Index of found item, or None next( (index for index in reversed( range( len( mylist ) ) ) if mylist[index] == item), None) # Alternately, reverse list and use "Find an item", but that copies the list revlist = mylist[::-1] if item in revlist: index = revlist.index( item ) # Sort by natural order # in-place sort mylist.sort() # Sort with custom comparator mylist = [ ('a', 10), ('b', 7), ('c',13), ('d',1) ] # sort by a key (sub-element mylist.sort( key = lambda item: item[1] ) # custom comparator def compare_fn( a, b ): return some_cond( a, b ) mylist.sort( key = functools.cmp_to_key( compare_fn ) ) 

# Split the list at arbitrary location tail_of_list = mylist[2:] head_of_list = mylist[:2] # Multiple splits based on a match mylist = ['a', 'b', 'c', 'd', 'b', 'e'] [list(y) for x, y in itertools.groupby( mylist, lambda z: z == 'b') if not x] # Clear the list mylist.clear() # Remove segment # delete from position 1 up to, but excluding position 3 del mylist[1:3] # Concatenate lists mylist + other_list # Insert list at location # list slicing replaces the segment of list with another one, here we replace a zero-length slice mylist[1:1] = other_list # Get a sublist # sublist starting at position 1 up to, but excluding, position 3 mylist[1:3] 

# Backward for item in reversed( mylist ): print( item ) # Partial segment iteration # using itertools.islice avoids copying the list (which is what would happen if you used a slice) for item in itertools.islice( mylist, 1, 4 ): print( item ) # Skipping elements # step from element 1 to 6 (exclusive) by 2 for item in itertools.islice( mylist, 1, 6, 2 ): print( item ) 

# Create from a static list of items mylist = [ 'abc', 'def', 'ghi'] # Create a range of numbers # a list of numbers from 10..20 (exclusive) numbers = list( range( 10, 20 ) ) 

# Mapping [number * 10 for number in numbers] # Filtering [number for number in numbers if number % 2 == 0] # Fold / Reduce # Summing up numbers using builtin add functools.reduce( operator.add, numbers ) # Joining string representations of items functools.reduce( lambda left,right: str( left ) + '/' + str( right ), mylist ) # Zip # the zip function produces a list of tuples zip( lista, listb ) # to alternate items into one list use reduce functools.reduce( operator.add, zip( lista, listb ) ) 

# Swap elements at two locations mylist[3], mylist[5] = mylist[5], mylist[3] # Reserve capacity # Python lists do not expose capacity  # Replace content in a list mylist[:] = other_list # Compare two lists lista == listb # Search a sorted list # bisect_left/bisect_right work with sorted lists, # find an item ndx using bisect_left, finds the left-most item ndx = bisect_left( numbers, 4 ) if ndx != len(numbers) and numbers[ndx] == 4 print( "Found at {}".format(ndx) ) # Iterators # Manually stepping through an iterator myiter = iter( mylist ) while True: try: n = next( myiter ) print(n) except StopIteration: break # Multiple iterators at the same time itera = iter( lista ) iterb = iter( listb ) while True: try: a = next( itera ) b = next( iterb ) print( a, b ) except StopIteration: break