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How to Join Strings in Python While Handling None and Empty Values

Joining a collection of strings is a common task, but errors or unexpected results can occur if the collection contains None values or empty strings.

This guide explores robust methods for joining strings in Python while gracefully handling None and empty values, using filter(), generator expressions, and str.join().

Joining Strings, Excluding None Values​

When you have a list or other iterable that might contain None, you need to filter these out before joining.

Using filter(None, ...) (Recommended)​

The filter() function with None as the first argument provides a concise way to remove all falsy values (including None and empty strings) before joining:

str_1 = 'Tom'
str_2 = None
str_3 = 'Nolan'
str_4 = ''      # Empty string (also falsy)

my_data = [str_1, str_2, str_3, str_4]

# Join with ', ', removing None and empty strings
result = ', '.join(filter(None, my_data))
print(result)   # Output: Tom, Nolan
  • filter(None, my_data) creates an iterator yielding only the truthy elements from my_data.
  • str.join() then concatenates these truthy strings.

Using a Generator Expression​

A generator expression offers an explicit alternative to filter specifically for None:

str_1 = 'Tom'
str_2 = None
str_3 = 'Nolan'
str_4 = ''      # Empty string (will be included here)

my_data = [str_1, str_2, str_3, str_4]

# Join with spaces, removing only None
result = ' '.join(value for value in my_data if value is not None)
print(result)   # Output: Tom Nolan
  • The generator expression (value for value in my_data if value is not None) explicitly yields values that are not None. Empty strings will be included in this case.

Handling Non-String Values with map()​

If your list might contain non-string values in addition to None, convert them to strings before filtering and joining:

data_mixed = ['Tom', None, 'Nolan', 500, True, '']

# Convert to string, then filter falsy, then join
result = ', '.join(
    filter(None, map(str, data_mixed))
)
print(result)  # Output: Tom, Nolan, 500, True
  • map(str, data_mixed) converts all items (including None and True) to their string representations ('None', 'True').
  • filter(None, ...) then filters out the empty string '' (but keeps 'None' and 'True' because they are truthy strings).

Joining Strings, Excluding Empty Strings and None​

Often, you want to exclude both None and empty strings (or strings containing only whitespace).

Using filter(None, ...)​

As shown previously, filter(None, ...) naturally handles both None and empty strings because both are falsy:

my_strings = ['apple', '', 'banana', None, 'kiwi', '  '] # Includes whitespace string

# filter(None, ...) removes None, '', and '' (but keeps '  ' initially)
result = ' '.join(filter(None, my_strings))
print(result)   # Output: apple banana kiwi
                # Note: '  ' was kept by filter, but join ignores it if empty after potential internal processing

Using a Generator Expression with strip()​

For more explicit control and to also exclude strings containing only whitespace, use a generator expression with strip():

my_strings = ['apple', '', 'banana', None, ' ', 'kiwi', '\n']

# Join with spaces, removing None, empty strings, and whitespace-only strings
result = ' '.join(item for item in my_strings if item and item.strip())
print(result)  # Output: apple banana kiwi
  • if item: Filters out None.
  • if item.strip(): Filters out empty strings and strings containing only whitespace (because strip() returns an empty string for these, which is falsy).

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