import urllib3
my_header_1={"User-Agent": "999999"}
ua="'User-Agent': '9999999999'"
my_header_2="{"+ua+"}"
http=urllib3.PoolManager()
r=http.request('GET', BASE_URL, headers=my_header_1) # все нормально выполняется

 r=http.request('GET', BASE_URL, headers=my_header_2) # выдает ошибку

     r=http.request('GET', BASE_URL, headers=my_header_2)
  File "/usr/lib/python3/site-packages/urllib3/request.py", line 69, in request
    **urlopen_kw)
  File "/usr/lib/python3/site-packages/urllib3/request.py", line 90, in request_encode_url
    return self.urlopen(method, url, **extra_kw)
  File "/usr/lib/python3/site-packages/urllib3/poolmanager.py", line 165, in urlopen
    response = conn.urlopen(method, u.request_uri, **kw)
  File "/usr/lib/python3/site-packages/urllib3/connectionpool.py", line 558, in urlopen
    headers = headers.copy()
AttributeError: 'str' object has no attribute 'copy'

 ua = "'User-Agent': '9999999999'"
my_header_2 = "{" + ua + "}"
print type(my_header_2)
<type 'str'>

 # -*- coding: utf-8 -*-
my_header_2 = {}
my_header_2['User-Agent'] = '9999999999'
my_header_2['User-HER'] = '0000000'
print type(my_header_2), my_header_2
<type 'dict'> {'User-HER': '0000000', 'User-Agent': '9999999999'}

 r.headers
HTTPHeaderDict({'Content-Length': '33', ...})

 {'Content-Length': '33', ...}
{} - словарь
[] - список
() - кортеж
"" - строка