diff --git a/searx/utils.py b/searx/utils.py
index d3969df58..23bcbc124 100644
--- a/searx/utils.py
+++ b/searx/utils.py
@@ -39,12 +39,17 @@ lang_to_lc_cache = dict()
 
 
 def searx_useragent():
+    """Return the searx User Agent"""
     return 'searx/{searx_version} {suffix}'.format(
            searx_version=VERSION_STRING,
            suffix=settings['outgoing'].get('useragent_suffix', ''))
 
 
 def gen_useragent(os=None):
+    """Return a random browser User Agent
+
+    See searx/data/useragents.json
+    """
     return str(useragents['ua'].format(os=os or choice(useragents['os']), version=choice(useragents['versions'])))
 
 
@@ -99,24 +104,40 @@ class HTMLTextExtractor(HTMLParser):
         return ''.join(self.result).strip()
 
 
-def html_to_text(html):
-    html = html.replace('\n', ' ')
-    html = ' '.join(html.split())
+def html_to_text(html_str):
+    """Extract text from a HTML string
+
+    Args:
+        * html_str (str): string HTML
+
+    Returns:
+        * str: extracted text
+
+    Examples:
+        >>> html_to_text('Example <span id="42">#2</span>')
+        'Example #2'
+
+        >>> html_to_text('<style>.span { color: red; }</style><span>Example</span>')
+        'Example'
+    """
+    html_str = html_str.replace('\n', ' ')
+    html_str = ' '.join(html_str.split())
     s = HTMLTextExtractor()
     try:
-        s.feed(html)
+        s.feed(html_str)
     except HTMLTextExtractorException:
-        logger.debug("HTMLTextExtractor: invalid HTML\n%s", html)
+        logger.debug("HTMLTextExtractor: invalid HTML\n%s", html_str)
     return s.get_text()
 
 
 def extract_text(xpath_results):
-    '''
-    if xpath_results is list, extract the text from each result and concat the list
-    if xpath_results is a xml element, extract all the text node from it
-    ( text_content() method from lxml )
-    if xpath_results is a string element, then it's already done
-    '''
+    """Extract text from a lxml result
+
+      * if xpath_results is list, extract the text from each result and concat the list
+      * if xpath_results is a xml element, extract all the text node from it
+        ( text_content() method from lxml )
+      * if xpath_results is a string element, then it's already done
+    """
     if type(xpath_results) == list:
         # it's list of result : concat everything using recursive call
         result = ''
@@ -135,29 +156,6 @@ def extract_text(xpath_results):
         return ' '.join(text.split())
 
 
-def extract_url(xpath_results, search_url):
-    if xpath_results == []:
-        raise Exception('Empty url resultset')
-    url = extract_text(xpath_results)
-
-    if url.startswith('//'):
-        # add http or https to this kind of url //example.com/
-        parsed_search_url = urlparse(search_url)
-        url = '{0}:{1}'.format(parsed_search_url.scheme or 'http', url)
-    elif url.startswith('/'):
-        # fix relative url to the search engine
-        url = urljoin(search_url, url)
-
-    # fix relative urls that fall through the crack
-    if '://' not in url:
-        url = urljoin(search_url, url)
-
-    # normalize url
-    url = normalize_url(url)
-
-    return url
-
-
 def normalize_url(url):
     parsed_url = urlparse(url)
 
@@ -178,7 +176,69 @@ def normalize_url(url):
     return url
 
 
+def extract_url(xpath_results, search_url):
+    """Extract and normalize URL from lxml Element
+
+    Args:
+        * xpath_results (Union[List[html.HtmlElement], html.HtmlElement]): lxml Element(s)
+        * search_url (str): Base URL
+
+    Example:
+        >>> def f(s, search_url):
+        >>>    return searx.utils.extract_url(html.fromstring(s), search_url)
+        >>> f('<span id="42">https://example.com</span>', 'http://example.com/')
+        'https://example.com/'
+        >>> f('https://example.com', 'http://example.com/')
+        'https://example.com/'
+        >>> f('//example.com', 'http://example.com/')
+        'http://example.com/'
+        >>> f('//example.com', 'https://example.com/')
+        'https://example.com/'
+        >>> f('/path?a=1', 'https://example.com')
+        'https://example.com/path?a=1'
+        >>> f('', 'https://example.com')
+        raise lxml.etree.ParserError
+        >>> searx.utils.extract_url([], 'https://example.com')
+        raise Exception
+
+    Raises:
+        * Exception
+        * lxml.etree.ParserError
+
+    Returns:
+        * str: normalized URL
+    """
+    if xpath_results == []:
+        raise Exception('Empty url resultset')
+    url = extract_text(xpath_results)
+
+    if url.startswith('//'):
+        # add http or https to this kind of url //example.com/
+        parsed_search_url = urlparse(search_url)
+        url = '{0}:{1}'.format(parsed_search_url.scheme or 'http', url)
+    elif url.startswith('/'):
+        # fix relative url to the search engine
+        url = urljoin(search_url, url)
+
+    # fix relative urls that fall through the crack
+    if '://' not in url:
+        url = urljoin(search_url, url)
+
+    # normalize url
+    url = normalize_url(url)
+
+    return url
+
+
 def dict_subset(d, properties):
+    """Extract a subset of a dict
+
+    Examples:
+        >>> dict_subset({'A': 'a', 'B': 'b', 'C': 'c'}, ['A', 'C'])
+        {'A': 'a', 'C': 'c'}
+        >>> >> dict_subset({'A': 'a', 'B': 'b', 'C': 'c'}, ['A', 'D'])
+        {'A': 'a'}
+    """
     result = {}
     for k in properties:
         if k in d:
@@ -186,8 +246,19 @@ def dict_subset(d, properties):
     return result
 
 
-# get element in list or default value
 def list_get(a_list, index, default=None):
+    """Get element in list or default value
+
+    Examples:
+        >>> list_get(['A', 'B', 'C'], 0)
+        'A'
+        >>> list_get(['A', 'B', 'C'], 3)
+        None
+        >>> list_get(['A', 'B', 'C'], 3, 'default')
+        'default'
+        >>> list_get(['A', 'B', 'C'], -1)
+        'C'
+    """
     if len(a_list) > index:
         return a_list[index]
     else:
@@ -195,6 +266,21 @@ def list_get(a_list, index, default=None):
 
 
 def get_torrent_size(filesize, filesize_multiplier):
+    """
+
+    Args:
+        * filesize (str): size
+        * filesize_multiplier (str): TB, GB, .... TiB, GiB...
+
+    Returns:
+        * int: number of bytes
+
+    Example:
+        >>> get_torrent_size('5', 'GB')
+        5368709120
+        >>> get_torrent_size('3.14', 'MiB')
+        3140000
+    """
     try:
         filesize = float(filesize)
 
@@ -221,14 +307,18 @@ def get_torrent_size(filesize, filesize_multiplier):
 
 
 def convert_str_to_int(number_str):
+    """Convert number_str to int or 0 if number_str is not a number."""
     if number_str.isdigit():
         return int(number_str)
     else:
         return 0
 
 
-# convert a variable to integer or return 0 if it's not a number
 def int_or_zero(num):
+    """Convert num to int or 0. num can be either a str or a list.
+    If num is a list, the first element is converted to int (or return 0 if the list is empty).
+    If num is a str, see convert_str_to_int
+    """
     if isinstance(num, list):
         if len(num) < 1:
             return 0
@@ -237,6 +327,22 @@ def int_or_zero(num):
 
 
 def is_valid_lang(lang):
+    """Return language code and name if lang describe a language.
+
+    Examples:
+        >>> is_valid_lang('zz')
+        False
+        >>> is_valid_lang('uk')
+        (True, 'uk', 'ukrainian')
+        >>> is_valid_lang(b'uk')
+        (True, 'uk', 'ukrainian')
+        >>> is_valid_lang('en')
+        (True, 'en', 'english')
+        >>> searx.utils.is_valid_lang('Español')
+        (True, 'es', 'spanish')
+        >>> searx.utils.is_valid_lang('Spanish')
+        (True, 'es', 'spanish')
+    """
     if isinstance(lang, bytes):
         lang = lang.decode()
     is_abbr = (len(lang) == 2)
@@ -264,8 +370,8 @@ def _get_lang_to_lc_dict(lang_list):
     return value
 
 
-# auxiliary function to match lang_code in lang_list
 def _match_language(lang_code, lang_list=[], custom_aliases={}):
+    """auxiliary function to match lang_code in lang_list"""
     # replace language code with a custom alias if necessary
     if lang_code in custom_aliases:
         lang_code = custom_aliases[lang_code]
@@ -287,8 +393,8 @@ def _match_language(lang_code, lang_list=[], custom_aliases={}):
     return _get_lang_to_lc_dict(lang_list).get(lang_code, None)
 
 
-# get the language code from lang_list that best matches locale_code
 def match_language(locale_code, lang_list=[], custom_aliases={}, fallback='en-US'):
+    """get the language code from lang_list that best matches locale_code"""
     # try to get language from given locale_code
     language = _match_language(locale_code, lang_list, custom_aliases)
     if language:
@@ -330,6 +436,7 @@ def load_module(filename, module_dir):
 
 
 def to_string(obj):
+    """Convert obj to its string representation."""
     if isinstance(obj, str):
         return obj
     if isinstance(obj, Number):
@@ -341,13 +448,19 @@ def to_string(obj):
 
 
 def ecma_unescape(s):
-    """
-    python implementation of the unescape javascript function
+    """Python implementation of the unescape javascript function
 
     https://www.ecma-international.org/ecma-262/6.0/#sec-unescape-string
     https://developer.mozilla.org/fr/docs/Web/JavaScript/Reference/Objets_globaux/unescape
+
+    Examples:
+        >>> ecma_unescape('%u5409')
+        '吉'
+        >>> ecma_unescape('%20')
+        ' '
+        >>> ecma_unescape('%F3')
+        'ó'
     """
-    # s = unicode(s)
     # "%u5409" becomes "吉"
     s = ecma_unescape4_re.sub(lambda e: chr(int(e.group(1), 16)), s)
     # "%20" becomes " ", "%F3" becomes "ó"
@@ -371,6 +484,11 @@ def get_engine_from_settings(name):
 
 
 def get_xpath(xpath_str):
+    """Return cached compiled XPath
+
+    There is no thread lock.
+    Worst case scenario, xpath_str is compiled more than one time.
+    """
     result = xpath_cache.get(xpath_str, None)
     if result is None:
         result = XPath(xpath_str)
@@ -379,5 +497,6 @@ def get_xpath(xpath_str):
 
 
 def eval_xpath(element, xpath_str):
+    """Equivalent of element.xpath(xpath_str) but compile xpath_str once for all."""
     xpath = get_xpath(xpath_str)
     return xpath(element)