osx - Python, Install Beautiful Soup4-4.1.3 on Mac. Access Denied? -

i can't figure out how install beautifulsoup4 on python. i've downloaded it, unpacked , moved user directory. typed in terminal 'cd beautifulsoup4-4.1.3' enter directory type 'python setup.py install' message:

    adams-imac:beautifulsoup4-4.1.3 adamparis$ python setup.py install running install running build running build_py creating build creating build/lib creating build/lib/bs4 copying bs4/__init__.py -> build/lib/bs4 copying bs4/dammit.py -> build/lib/bs4 copying bs4/element.py -> build/lib/bs4 copying bs4/testing.py -> build/lib/bs4 creating build/lib/bs4/builder copying bs4/builder/__init__.py -> build/lib/bs4/builder copying bs4/builder/_html5lib.py -> build/lib/bs4/builder copying bs4/builder/_htmlparser.py -> build/lib/bs4/builder copying bs4/builder/_lxml.py -> build/lib/bs4/builder creating build/lib/bs4/tests copying bs4/tests/__init__.py -> build/lib/bs4/tests copying bs4/tests/test_builder_registry.py -> build/lib/bs4/tests copying bs4/tests/test_docs.py -> build/lib/bs4/tests copying bs4/tests/test_html5lib.py -> build/lib/bs4/tests copying bs4/tests/test_htmlparser.py -> build/lib/bs4/tests copying bs4/tests/test_lxml.py -> build/lib/bs4/tests copying bs4/tests/test_soup.py -> build/lib/bs4/tests copying bs4/tests/test_tree.py -> build/lib/bs4/tests running install_lib creating /library/python/2.7/site-packages/bs4 error: not create '/library/python/2.7/site-packages/bs4': permission denied adams-imac:beautifulsoup4-4.1.3 adamparis$  

at end says 'permission denied. why this?

try $ sudo python setup.py install. type in password when prompted.