expression ['abc'[( )([ ] )] syntactically nested objects are internally represented as references ( pointersto separate pieces of memory changing mutable object in place may affect other references to the same object elsewhere in your program if you don' want such behavioryou'll need to tell python to copy the object explicitly we studied this phenomenon in but it can become more subtle when larger objects of the sort we've explored since then come into play for instancethe following example creates list assigned to xand another list assigned to that embeds reference back to list it also creates dictionary that contains another reference back to list xx [ [' ' ' ' {' ': ' ': embed references to ' object at this pointthere are three references to the first list createdfrom the name xfrom inside the list assigned to land from inside the dictionary assigned to the situation is illustrated in figure - because lists are mutablechanging the shared list object from any of the three references also changes what the other two referencex[ 'surprisechanges all three referencesl [' '[ 'surprise' ]' ' {' '[ 'surprise' ]' ' references are higher-level analog of pointers in other languages that are always followed when used although you can' grab hold of the reference itselfit' possible to tuplesfilesand everything else