How do I escape curly-brace ({}) characters in a string while using .format (or an f-string)?

You need to double the {{ and }}:

>>> x = " {{ Hello }} {0} "
>>> print(x.format(42))
' { Hello } 42 '

Here's the relevant part of the Python documentation for format string syntax:

Format strings contain “replacement fields” surrounded by curly braces {}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output. If you need to include a brace character in the literal text, it can be escaped by doubling: {{ and }}.

Python 3.6+ (2017)

In the recent versions of Python one would use f-strings (see also PEP498).

With f-strings one should use double {{ or }}

n = 42  
print(f" {{Hello}} {n} ")

produces the desired

 {Hello} 42

If you need to resolve an expression in the brackets instead of using literal text you'll need three sets of brackets:

hello = "HELLO"
print(f"{{{hello.lower()}}}")

produces

{hello}

You escape it by doubling the braces.

Eg:

x = "{{ Hello }} {0}"
print(x.format(42))

You want to format a string with the character { or }

You just have to double them.

format { with f'{{' and }with f'}}'

So :

name = "bob"
print(f'Hello {name} ! I want to print }} and {{ or {{ }}')

Output :

Hello bob ! I want to print } and { or { }

OR for the exact example :

number = 42
print(f'{{Hello}} {number}')

Will print :

{Hello} 42

Finally :

number = 42
string = "bob"
print(f'{{Hello}} {{{number}}} {number} {{{string}}} {string} ')

{Hello} {42} 42 {bob} bob

The OP wrote this comment:

I was trying to format a small JSON for some purposes, like this: '{"all": false, "selected": "{}"}'.format(data) to get something like {"all": false, "selected": "1,2"}

It's pretty common that the "escaping braces" issue comes up when dealing with JSON.

I suggest doing this:

import json
data = "1,2"
mydict = {"all": "false", "selected": data}
json.dumps(mydict)

It's cleaner than the alternative, which is:

'{{"all": false, "selected": "{}"}}'.format(data)

Using the json library is definitely preferable when the JSON string gets more complicated than the example.

Try this:

x = "{{ Hello }} {0}"

Try doing this:

x = " {{ Hello }} {0} "
print x.format(42)

Although not any better, just for the reference, you can also do this:

>>> x = '{}Hello{} {}'
>>> print x.format('{','}',42)
{Hello} 42

It can be useful for example when someone wants to print {argument}. It is maybe more readable than '{{{}}}'.format('argument')

Note that you omit argument positions (e.g. {} instead of {0}) after Python 2.7

key = "FOOBAR"
print(f"hello {{{key}}}")

outputs

hello {FOOBAR}

In case someone wanted to print something inside curly brackets using fstrings.

f-strings (python 3)

You can avoid having to double the curly brackets by using f-strings ONLY for the parts of the string where you want the f-magic to apply, and using regular (dumb) strings for everything that is literal and might contain 'unsafe' special characters. Let python do the string joining for you simply by stacking multiple strings together.

number = 42
print(" { Hello }"  
f" {number} " 
"{ thanks for all the fish }")

### OUTPUT:
{ Hello } 42 { thanks for all the fish }

NOTE: Line breaks between the strings are NOT required. I have only added them for readability. You could as well write the code above as shown below:

^{⚠️ WARNING: This might hurt your eyes or make you dizzy!}

print("{Hello}"f"{number}""{thanks for all the fish}")

If you need to keep two curly braces in the string, you need 5 curly braces on each side of the variable.

>>> myvar = 'test'
>>> "{{{{{0}}}}}".format(myvar)
'{{test}}'

If you are going to be doing this a lot, it might be good to define a utility function that will let you use arbitrary brace substitutes instead, like

def custom_format(string, brackets, *args, **kwargs):
    if len(brackets) != 2:
        raise ValueError('Expected two brackets. Got {}.'.format(len(brackets)))
    padded = string.replace('{', '{{').replace('}', '}}')
    substituted = padded.replace(brackets[0], '{').replace(brackets[1], '}')
    formatted = substituted.format(*args, **kwargs)
    return formatted

>>> custom_format('{{[cmd]} process 1}', brackets='[]', cmd='firefox.exe')
'{{firefox.exe} process 1}'

Note that this will work either with brackets being a string of length 2 or an iterable of two strings (for multi-character delimiters).

I recently ran into this, because I wanted to inject strings into preformatted JSON. My solution was to create a helper method, like this:

def preformat(msg):
    """ allow {{key}} to be used for formatting in text
    that already uses curly braces.  First switch this into
    something else, replace curlies with double curlies, and then
    switch back to regular braces
    """
    msg = msg.replace('{{', '<<<').replace('}}', '>>>')
    msg = msg.replace('{', '{{').replace('}', '}}')
    msg = msg.replace('<<<', '{').replace('>>>', '}')
    return msg

You can then do something like:

formatted = preformat("""
    {
        "foo": "{{bar}}"
    }""").format(bar="gas")

Gets the job done if performance is not an issue.

Reason is , {} is the syntax of .format() so in your case .format() doesn't recognize {Hello} so it threw an error.

you can override it by using double curly braces {{}},

x = " {{ Hello }} {0} "

or

try %s for text formatting,

x = " { Hello } %s"
print x%(42)  

I am ridiculously late to this party. I am having success placing the brackets in the replacement element, like this:

print('{0} {1}'.format('{hello}', '{world}'))

which prints

{hello} {world}

Strictly speaking this is not what OP is asking, as s/he wants the braces in the format string, but this may help someone.

I stumbled upon this problem when trying to print text, which I can copy paste into a Latex document. I extend on this answer and make use of named replacement fields:

Lets say you want to print out a product of mulitple variables with indices such as , which in Latex would be $A_{ 0042 }*A_{ 3141 }*A_{ 2718 }*A_{ 0042 }$ The following code does the job with named fields so that for many indices it stays readable:

idx_mapping = {'i1':42, 'i2':3141, 'i3':2178 }
print('$A_{{ {i1:04d} }} * A_{{ {i2:04d} }} * A_{{ {i3:04d} }} * A_{{ {i1:04d} }}$'.format(**idx_mapping))

You can use a "quote wall" to separate the formatted string part from the regular string part.

From:

print(f"{Hello} {42}")

to

print("{Hello}"f" {42}")

A clearer example would be

string = 10
print(f"{string} {word}")

Output:

NameError: name 'word' is not defined

Now, add the quote wall like so:

string = 10
print(f"{string}"" {word}")

Output:

10 {word}

I used a double {{ }} to prevent fstring value injection,

for example, heres my Postgres UPDATE statement to update a integer array column that takes expression of {} to capture the array, ie:

ports = '{100,200,300}'

with fstrings its,

ports = [1,2,3]

query = f"""
   UPDATE table SET ports = '{{{ports}}}' WHERE id = 1
"""

the actual query statement will be,

UPDATE table SET ports = '{1,2,3}'

which is a valid postgres satement

If you want to print just one side of the curly brace:

a=3
print(f'{"{"}{a}')
>>> {3

If you want to only print one curly brace (for example {) you can use {{, and you can add more braces later in the string if you want. For example:

>>> f'{{ there is a curly brace on the left. Oh, and 1 + 1 is {1 + 1}'
'{ there is a curly brace on the left. Oh, and 1 + 1 is 2'

When you're just trying to interpolate code strings I'd suggest using jinja2 which is a full-featured template engine for Python, ie:

from jinja2 import Template

foo = Template('''
#include <stdio.h>

void main() {
    printf("hello universe number {{number}}");
}
''')

for i in range(2):
    print(foo.render(number=i))

So you won't be enforced to duplicate curly braces as the whole bunch of other answers suggest

If you need curly braces within a f-string template that can be formatted, you need to output a string containing two curly braces within a set of curly braces for the f-string:

css_template = f"{{tag}} {'{{'} margin: 0; padding: 0;{'}}'}"
for_p = css_template.format(tag="p")
# 'p { margin: 0; padding: 0;}'

Or just parametrize the bracket itself? Probably very verbose.

x = '{open_bracket}42{close_bracket}'.format(open_bracket='{', close_bracket='}') 
print(x)
# {42}